The contrast between the colors for the background and the text in the Tooltip of the Chart series is very low (dark background and black text). https://www.screencast.com/t/fhx6937j

The problem is most obvious in the Outlook, Simple, Web20 and Sunset skins. 

Workaround would be  setting the Color property for the ToolTipAppearance to White

For the time being you can set the types through the kendoWidget:
		<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
			<script>
				function pageLoad() {
					var chart = $find('<%=RadHtmlChart1.ClientID%>');
					chart._chartObject.options.series[0].type = "verticalLine";
					chart.repaint();
				}
			</script>
		</telerik:RadCodeBlock>
		<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="600px" Height="400px">
			<PlotArea>
				<Series>
					<telerik:LineSeries Name="Product 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="15000" />
							<telerik:CategorySeriesItem Y="23000" />
							<telerik:CategorySeriesItem Y="10000" />
						</SeriesItems>
					</telerik:LineSeries>
				</Series>
				<XAxis>
					<Items>
						<telerik:AxisItem LabelText="1" />
						<telerik:AxisItem LabelText="2" />
						<telerik:AxisItem LabelText="3" />
					</Items>
				</XAxis>
			</PlotArea>
			<ChartTitle Text="Product sales for 2011">
			</ChartTitle>
			<Legend>
				<Appearance Position="Bottom" />
			</Legend>
		</telerik:RadHtmlChart>

For the time being the following workaround can be used:

JavaScript:

    <script>
        function pageLoad() {
            var chart = $find("<%=RadHtmlChart1.ClientID%>");
            chart._chartObject.options.series[0].labels.background = "yellow";
            chart._chartObject.options.series[0].labels.border = {
                width: 2,
                dashType: "dashDot",
                color: "green"
            };
            chart.repaint();
        }
    </script>

ASPX:

        <telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="400">
            <PlotArea>
                <Series>
                    <telerik:ColumnSeries>
                        <SeriesItems>
                            <telerik:CategorySeriesItem Y="10" />
                            <telerik:CategorySeriesItem Y="30" />
                            <telerik:CategorySeriesItem Y="20" />
                            <telerik:CategorySeriesItem Y="25" />
                        </SeriesItems>
                    </telerik:ColumnSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>

For the time being the property can be set through the underlying Kendo Chart widget. For example:

		<script>
			function OnLoad(chart) {
				var widget = chart.get_kendoWidget();
				//also applies for the minor grid lines - replace majorGridLines with minorGridLines
				//Numeric series
				widget.options.xAxis.majorGridLines.step = 5;
				widget.options.yAxis.majorGridLines.step = 5;
				//Category series
				//widget.options.categoryAxis.majorGridLines.step = 5;
				//widget.options.valueAxis.majorGridLines.step = 5;
				widget.redraw();
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="BubbleChart" Width="500" Height="400">
			<ClientEvents OnLoad="OnLoad" />
			<ChartTitle Text="Market Share Study">
			</ChartTitle>
			<PlotArea>
				<Appearance>
					<FillStyle BackgroundColor="White"></FillStyle>
				</Appearance>
				<XAxis MinValue="0" MaxValue="100" Step="10">
					<MinorGridLines Visible="false" />
				</XAxis>
				<YAxis MinValue="0" MaxValue="100" Step="10">
					<MinorGridLines Visible="false" />
				</YAxis>
				<Series>
					<telerik:BubbleSeries>
						<Appearance FillStyle-BackgroundColor="#6ab2c9">
						</Appearance>
						<TooltipsAppearance DataFormatString="Percentage of Market Share: {2}%<br /> Number of products: {0}<br /> Sales: ${1}" />
						<SeriesItems>
							<telerik:BubbleSeriesItem Size="3" X="5" Y="55" />
							<telerik:BubbleSeriesItem Size="12" X="14" Y="80" />
							<telerik:BubbleSeriesItem Size="33" X="20" Y="60" />
							<telerik:BubbleSeriesItem Size="10" X="18" Y="24" />
							<telerik:BubbleSeriesItem Size="42" X="22" Y="32" />
						</SeriesItems>
					</telerik:BubbleSeries>
				</Series>
			</PlotArea>
			<Legend>
				<Appearance Position="Right"></Appearance>
			</Legend>
		</telerik:RadHtmlChart>

For the time being the function can be set through the kendoWidget:

<telerik:RadHtmlChart runat="server" ID="ColumnChart1" Transitions="true">
    <PlotArea>
        <Series>
            <telerik:ColumnSeries Name="Product 1">
                <TooltipsAppearance BackgroundColor="Orange" DataFormatString="{0} sales" />
                <SeriesItems>
                    <telerik:CategorySeriesItem Y="15000" />
                    <telerik:CategorySeriesItem Y="23000" />
                    <telerik:CategorySeriesItem Y="10000" />
                    <telerik:CategorySeriesItem Y="16000" />
                </SeriesItems>
            </telerik:ColumnSeries>
        </Series>
        <XAxis AxisCrossingValue="0" Color="Black" MajorTickType="Outside" MinorTickType="Outside"
            Reversed="false">
            <Items>
                <telerik:AxisItem LabelText="1" />
                <telerik:AxisItem LabelText="2" />
                <telerik:AxisItem LabelText="3" />
                <telerik:AxisItem LabelText="4" />
            </Items>
            <LabelsAppearance DataFormatString="Q{0}" RotationAngle="0" />
            <MajorGridLines Color="#EFEFEF" Width="1" />
            <MinorGridLines Color="#F7F7F7" Width="1" />
            <TitleAppearance Position="Center" RotationAngle="0" Text="Quarters" />
        </XAxis>
        <YAxis AxisCrossingValue="0" Color="Black" MajorTickSize="1" MajorTickType="Outside"
            MaxValue="50000" MinorTickSize="1" MinorTickType="Outside" MinValue="0" Reversed="false"
            Step="10000">
            <LabelsAppearance DataFormatString="{0} sales" RotationAngle="0" />
            <TitleAppearance Position="Center" RotationAngle="0" Text="Sales" />
        </YAxis>
    </PlotArea>
    <ChartTitle Text="Product Sales for 2011">
    </ChartTitle>
    <Legend>
        <Appearance Position="Bottom" />
    </Legend>
</telerik:RadHtmlChart>
<script>
    function pageLoad() {
        $ = $telerik.$;
        var kendoWidget = $find("<%=ColumnChart1.ClientID%>").get_kendoWidget();
        kendoWidget.options.series[0].labels.position = function (point) {
            if (point.value < 20000) {
                console.log(1);
                return "insideEnd";
            } else {
                console.log(2);
                return "outsideEnd";
            }
        }
        kendoWidget.redraw();
    }
</script>

A good improvement in the RadHtmlChart would be to add MaxSize and MinSize properties for the series, which would allow the control the scaling of the bubble chart item. The property can be set through the chartObject:

        <script>
            function pageLoad() {
                var chart = $find("<%=BubbleChart.ClientID%>");
                chart.get_kendoWidget().options.series[0].minSize = 1;
                chart.get_kendoWidget().options.series[0].maxSize = 10;
                chart.get_kendoWidget().redraw();
            }
        </script>
        ASPX:
        <telerik:RadHtmlChart runat="server" ID="BubbleChart" Width="500" Height="400">
            <PlotArea>
                <Series>
                    <telerik:BubbleSeries>
                        <Appearance FillStyle-BackgroundColor="#6ab2c9">
                        </Appearance>
                        <SeriesItems>
                            <telerik:BubbleSeriesItem Size="1" X="5" Y="5500" />
                            <telerik:BubbleSeriesItem Size="5" X="14" Y="12200" />
                            <telerik:BubbleSeriesItem Size="15" X="20" Y="39000" />
                        </SeriesItems>
                    </telerik:BubbleSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>


More detailed information regarding these properties can be found in the API reference of the Kendo UI Chart here:
http://docs.telerik.com/kendo-ui/api/javascript/dataviz/ui/chart#configuration-series.maxSize

For the time being the function can be set through the kendoWidget:
ASPX:
		<telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
		<script>
			function pageLoad() {
				var chart = $find("<%=RadHtmlChart1.ClientID%>");
				chart._chartObject.options.series[0].markers.type = function (point) {
					if (point.value > 2) {
						return "triangle";
					}
					else {
						return "circle";
					}
				};
				chart.repaint();
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="600px" Height="400px">
			<PlotArea>
				<Series>
					<telerik:LineSeries Name="Product 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="1" />
							<telerik:CategorySeriesItem Y="2" />
							<telerik:CategorySeriesItem Y="3" />
						</SeriesItems>
					</telerik:LineSeries>
				</Series>
				<XAxis>
					<Items>
						<telerik:AxisItem LabelText="1" />
						<telerik:AxisItem LabelText="2" />
						<telerik:AxisItem LabelText="3" />
					</Items>
				</XAxis>
			</PlotArea>
			<ChartTitle Text="Product sales for 2011">
			</ChartTitle>
			<Legend>
				<Appearance Position="Bottom" />
			</Legend>
		</telerik:RadHtmlChart>

When you configure RadHtmlChart's YAxis with NarrowRange set to true, the setting is not serialized and as a result, not applied.
You can avoid it by setting the narrowRange on the client-side:

		<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="250px" Height="300px">
			<ClientEvents OnLoad="chartLoad" />
			...
		</telerik:RadHtmlChart>
		<script>
			function chartLoad(){
				var chart = $telerik.$("#RadHtmlChart1").data("kendoChart");
				chart.options.valueAxis.narrowRange = true;
			}
		</script>

 Workaround use padding instead of margin:
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server"  Width="600px" Height="400px">
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Product 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="15000" />
							<telerik:CategorySeriesItem Y="-4000" />
							<telerik:CategorySeriesItem Y="10000" />
						</SeriesItems>
					</telerik:ColumnSeries>
				</Series>
				<XAxis>
					<LabelsAppearance RotationAngle="45">
						<TextStyle Padding="135 0 0 0" />
					</LabelsAppearance>
					<Items>
						<telerik:AxisItem LabelText="item1" />
						<telerik:AxisItem LabelText="item2" />
						<telerik:AxisItem LabelText="item3" />
					</Items>
				</XAxis>
			</PlotArea>
		</telerik:RadHtmlChart>

The functionality is similar to the one in Kendo UI Chart (http://demos.telerik.com/kendo-ui/dataviz/line-charts/notes.html) :
http://dojo.telerik.com/OZuNU

For the time being you can try to utilize the kendo.drawing API in order to place the text on a particular position.  This forum may be helpful on the matter - http://www.telerik.com/forums/custom-drawing#sOkmYWbfrUW404Rr13eLDQ

Currently, the HighlightAppearance of the Series does not have a property that controls the InactiveOpacity:

• https://docs.telerik.com/kendo-ui/api/javascript/dataviz/ui/chart/configuration/series.highlight#serieshighlightinactiveopacity 

As a temporary workaround, this can be set through the options in the OnKendoWidgetInitializing event:

<script>
    function OnKendoWidgetInitializing(sender, args) {
        var series = args.series[0]
        series.highlight = series.highlight || {};
        series.highlight.inactiveOpacity = 0.2
    }
</script>
<telerik:RadHtmlChart ID="RadHtmlChart5" runat="server" BorderColor="Fuchsia" BorderStyle="Solid" BorderWidth="1px">
    <ClientEvents OnKendoWidgetInitializing="OnKendoWidgetInitializing" />
    <PlotArea>

        <Series>
            <telerik:PieSeries StartAngle="90">
                <%-- This is how it should be configured after the feature request --%>
                <%--<HighlightAppearance InactiveOpacity="0.2" />--%>

                <SeriesItems>
                    <telerik:PieSeriesItem Name="Slice1" Y="60"></telerik:PieSeriesItem>
                    <telerik:PieSeriesItem Name="Slice2" Y="40"></telerik:PieSeriesItem>
                </SeriesItems>
            </telerik:PieSeries>
        </Series>
    </PlotArea>
</telerik:RadHtmlChart>

Width and height properties for the legend that will provide firm dimensions in pixels, so regardless of the series names' length, it will always have constant size.
You can set width and height through the kendo widget:
		<script>
			function pageLoad() {
				var chart = $find("<%=DonutChart.ClientID%>").get_kendoWidget();
				chart.options.legend.height = 80;
				chart.redraw();
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="DonutChart" Width="500" Height="500" Transitions="true">
			<Appearance>
				<FillStyle BackgroundColor="White"></FillStyle>
			</Appearance>
			<ChartTitle Text="OS Usage Statistics for December 2012">
				<Appearance Align="Center" Position="Top"></Appearance>
			</ChartTitle>
			<PlotArea>
				<Series>
					<telerik:DonutSeries>
						<LabelsAppearance Visible="false">
						</LabelsAppearance>
						<TooltipsAppearance DataFormatString="{0}%" BackgroundColor="White"></TooltipsAppearance>
						<SeriesItems>
							<telerik:PieSeriesItem BackgroundColor="#00adcc" Name="Win7"
								Y="55.6"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#cccccc" Name="Win8" Y="2.5"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#999999" Name="Vista" Y="2.8"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#888888" Name="NT" Y="1.8"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#777777" Name="WinXP" Y="21.1"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#666666" Name="Linux" Y="4.7"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#555555" Name="Mac" Y="8.7"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#444444" Name="Mobile" Y="2.2"></telerik:PieSeriesItem>
						</SeriesItems>
					</telerik:DonutSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>

Currently tooltips expose properties for changing the background color, format and visibility. Add more appearance options like color (Color property is already exposed), font, border width, border color and padding. For the time being the following workaround can be used:
	<style type="text/css">
		.k-tooltip
		{
			/*Sets border width and color*/
			border-width: 5px !important;
			border-color: Green !important;
			/*Sets padding*/
			padding-left: 5px !important;
			padding-right: 5px !important;
			/*Sets FontSize, FontFamily and color*/
			font: 15px Arial,Helvetica,sans-serif !important;
			color: Red !important;
		}
	</style>

When using custom shapes (circles, triangles, etc) in a RadHtmlChart, the custom shape should be displayed in the legend instead of always a square.  This is because when printing with a black & white printer, the colored squares won't help the reader read the chart, however the shapes would.

For the time being the property (http://docs.telerik.com/kendo-ui/api/javascript/dataviz/ui/chart#configuration-legend.border) can be set through the chartObject. For example:
JavaScript:
	<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
		<script>
			function pageLoad() {
				var chart = $find("<%=PieChart1.ClientID%>");
				chart._chartObject.options.legend.border = { width: 2, color: "green", dashType: "longDashDotDot" }
				chart.repaint();
			}
		</script>
	</telerik:RadCodeBlock>
ASPX:
		<telerik:RadHtmlChart runat="server" ID="PieChart1" Transitions="true">
			<PlotArea>
				<Series>
					<telerik:PieSeries StartAngle="90">
						<LabelsAppearance Position="OutsideEnd" DataFormatString="{0} %" />
						<TooltipsAppearance DataFormatString="{0} %" />
						<SeriesItems>
							<telerik:PieSeriesItem BackgroundColor="Purple" Exploded="true" Name="Internet Explorer" Y="18.3" />
							<telerik:PieSeriesItem BackgroundColor="Orange" Exploded="false" Name="Firefox" Y="35.8" />
							<telerik:PieSeriesItem BackgroundColor="Green" Exploded="false" Name="Chrome" Y="38.3" />
							<telerik:PieSeriesItem BackgroundColor="Blue" Exploded="false" Name="Safari" Y="4.5" />
							<telerik:PieSeriesItem BackgroundColor="Red" Exploded="false" Name="Opera" Y="2.3" />
						</SeriesItems>
					</telerik:PieSeries>
				</Series>
			</PlotArea>
			<ChartTitle Text="Browser Usage for April 2012">
			</ChartTitle>
		</telerik:RadHtmlChart>

A possible workaround: 
	<form id="form1" runat="server">
		<telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
		<script>
			var ExplodeLevel = 0.15;
		</script>
		<telerik:RadHtmlChart runat="server" ID="PieChart1" Transitions="true">
			<PlotArea>
				<Series>
					<telerik:PieSeries StartAngle="90">
						<LabelsAppearance Position="OutsideEnd" DataFormatString="{0} %" />
						<TooltipsAppearance DataFormatString="{0} %" />
						<SeriesItems>
							<telerik:PieSeriesItem BackgroundColor="Purple" Exploded="true" Name="Internet Explorer" Y="18.3" />
							<telerik:PieSeriesItem BackgroundColor="Orange" Exploded="false" Name="Firefox" Y="35.8" />
							<telerik:PieSeriesItem BackgroundColor="Green" Exploded="false" Name="Chrome" Y="38.3" />
							<telerik:PieSeriesItem BackgroundColor="Blue" Exploded="true" Name="Safari" Y="4.5" />
							<telerik:PieSeriesItem BackgroundColor="Red" Exploded="false" Name="Opera" Y="2.3" />
						</SeriesItems>
					</telerik:PieSeries>
				</Series>
			</PlotArea>
			<ChartTitle Text="Browser Usage for April 2012">
			</ChartTitle>
		</telerik:RadHtmlChart>
		<script>
			var math = Math,
				dataviz = kendo.dataviz,
				Box2D = dataviz.Box2D,
				Point2D = dataviz.Point2D,
				OUTSIDE_END = "outsideEnd",
				RIGHT = "right",
				util = kendo.util,
				valueOrDefault = util.valueOrDefault;

			kendo.dataviz.PieChart.fn.reflow = function (targetBox) {
				var chart = this,
					options = chart.options,
					box = targetBox.clone(),
					space = 5,
					minWidth = math.min(box.width(), box.height()),
					halfMinWidth = minWidth / 2,
					defaultPadding = minWidth - minWidth * 0.85,
					padding = valueOrDefault(options.padding, defaultPadding),
					newBox = Box2D(box.x1, box.y1,
					box.x1 + minWidth, box.y1 + minWidth),
					newBoxCenter = newBox.center(),
					seriesConfigs = chart.seriesConfigs || [],
					boxCenter = box.center(),
					points = chart.points,
					count = points.length,
					seriesCount = options.series.length,
					leftSideLabels = [],
					rightSideLabels = [],
					seriesConfig, seriesIndex, label,
					segment, sector, r, i, c;

				padding = padding > halfMinWidth - space ? halfMinWidth - space : padding;
				newBox.translate(boxCenter.x - newBoxCenter.x, boxCenter.y - newBoxCenter.y);
				r = halfMinWidth - padding;
				c = Point2D(
					r + newBox.x1 + padding,
					r + newBox.y1 + padding
				);

				for (i = 0; i < count; i++) {
					segment = points[i];

					sector = segment.sector;
					sector.r = r;
					sector.c = c;
					seriesIndex = segment.seriesIx;
					if (seriesConfigs.length) {
						seriesConfig = seriesConfigs[seriesIndex];
						sector.ir = seriesConfig.ir;
						sector.r = seriesConfig.r;
					}

					if (seriesIndex == seriesCount - 1 && segment.explode) {
						sector.c = sector.clone().radius(sector.r * ExplodeLevel).point(sector.middle());
					}

					segment.reflow(newBox);

					label = segment.label;
					if (label) {
						if (label.options.position === OUTSIDE_END) {
							if (seriesIndex == seriesCount - 1) {
								if (label.orientation === RIGHT) {
									rightSideLabels.push(label);
								} else {
									leftSideLabels.push(label);
								}
							}
						}
					}
				}

				if (leftSideLabels.length > 0) {
					leftSideLabels.sort(chart.labelComparator(true));
					chart.leftLabelsReflow(leftSideLabels);
				}

				if (rightSideLabels.length > 0) {
					rightSideLabels.sort(chart.labelComparator(false));
					chart.rightLabelsReflow(rightSideLabels);
				}

				chart.box = newBox;
			};
		</script>
	</form>

For the time being the following workaround can be used:
JavaScript:
	<script>
		function pageLoad() {
			var pieChart = $find('<%=PieChart1.ClientID%>');
			pieChart._chartObject.options.seriesDefaults.labels.distance = 10;
			pieChart.repaint();
		}
	</script>
ASPX:
		<telerik:RadHtmlChart runat="server" ID="PieChart1" Width="300" Height="300">
			<PlotArea>
				<Series>
					<telerik:PieSeries>
						<SeriesItems>
							<telerik:PieSeriesItem Name="Item 1" Y="18.3"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem Name="Item 2" Y="35.8"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem Name="Item 3" Y="38.3"></telerik:PieSeriesItem>
						</SeriesItems>
					</telerik:PieSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>