Similar to the feature in RadChart - http://demos.telerik.com/aspnet-ajax/chart/examples/newfeatures/logarithmicaxis/defaultcs.aspx. 
For the time being the following workaround can be used:
ASPX:
		<script>
			function pageLoad() {
				var chart = $find("<%=RadHtmlChart1.ClientID%>");
				chart._chartObject.options.valueAxis.type = "log";
				chart.repaint();
			}
		</script>
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600px" Height="400px">
			<PlotArea>
				<YAxis></YAxis>
				<Series>
					<telerik:ColumnSeries>
					</telerik:ColumnSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>
C#:
	protected void Page_Load(object sender, EventArgs e)
	{
		double[] yValues = new double[] { 0.01, 0.05, 0.11, 2, 2.5, 10, 30 };
		for (int i = 0; i < yValues.Length; i++)
		{
			CategorySeriesItem csi1 = new CategorySeriesItem() { Y = (decimal)yValues[i] };
			(RadHtmlChart1.PlotArea.Series[0] as ColumnSeries).SeriesItems.Add(csi1);
		}
	} 

Currenlty Series like Bubble, Bar, Column, Donut, Pie and Candlestick do not expose any appearance properties for the Series border. Expose appearance border properties like width, dashtype, color and opacity.

Temporary workaround for managing the border appearance:
Javascript:
	<script language="javascript">
		function pageLoad() {
			var chart = $find("RadHtmlChart1");
			chart._chartObject.options.series[0].border = {
				width: 2,
				color: "black",
				dashType: "dash",
				opacity: 0.2
			}
			chart.repaint();
		}
	</script>

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

A good improvement in the RadHtmlChart would be to add an Opacity property for the series, which would allow the control of its transparency. The property can be set through the chartObject:
JavaScript:
		<script>
			function pageLoad() {
				var chart = $find("<%=BubbleChart.ClientID%>");
				chart._chartObject.options.series[0].opacity = 0.3;
				chart.repaint();
			}
		</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="3" X="5" Y="5500" />
							<telerik:BubbleSeriesItem Size="12" X="14" Y="12200" />
							<telerik:BubbleSeriesItem Size="33" X="20" Y="39000" />
							<telerik:BubbleSeriesItem Size="10" X="18" Y="24400" />
							<telerik:BubbleSeriesItem Size="42" X="20" Y="32000" />
						</SeriesItems>
					</telerik:BubbleSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>

Similar to the RadChart's http://demos.telerik.com/aspnet-ajax/chart/examples/newfeatures/intelligentlabels/defaultcs.aspx For the time being you can implement a custom logic for offsetting y point positions of labels. For example offset every 2nd item label position:
       <telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
		<script>
			function pageLoad() {
				offsetItemLabels();
			}
			function offsetItemLabels() {
				//Find all text elements on the page
				var textElements = document.getElementsByTagName("text");
				var numSeriesItemLabels = 0;
				//Iterate through the text elements
				for (var i = 0; i < textElements.length; i++) {
					var currTextEl = textElements[i];
					//Set the desired string/symbol that represents the series labels
					var SeriesLabelSymbol = "@SL";
					if (currTextEl.textContent.indexOf(SeriesLabelSymbol) != -1) {
						numSeriesItemLabels++;
						var currText = currTextEl.textContent.split(SeriesLabelSymbol);
						//Offset y point for every 2nd item label
						currTextEl.textContent = currText[1];
						if ((numSeriesItemLabels) % 2 == 0) {
							currTextEl.attributes.y.textContent = currTextEl.attributes.y.textContent * 1 + 35;
						}
					}
				}
			}
		</script>
        <telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600px" Height="400px">
            <ChartTitle Text="Chart title">
            </ChartTitle>
            <PlotArea>
                <Series>
                    <telerik:LineSeries Name="Series 1">
                        <SeriesItems>
                            <telerik:CategorySeriesItem Y="18" />
                            <telerik:CategorySeriesItem Y="18" />
                            <telerik:CategorySeriesItem Y="18" />
                            <telerik:CategorySeriesItem Y="18" />
                        </SeriesItems>
                        <LabelsAppearance>
                            <ClientTemplate>
@SLCategory is #=category# Value is: #=value#
                            </ClientTemplate>
                        </LabelsAppearance>
                    </telerik:LineSeries>
                </Series>
                <XAxis>
                    <Items>
                        <telerik:AxisItem LabelText="item 1" />
                        <telerik:AxisItem LabelText="item 2" />
                        <telerik:AxisItem LabelText="item 3" />
                        <telerik:AxisItem LabelText="item 4" />
                    </Items>
                </XAxis>
            </PlotArea>
        </telerik:RadHtmlChart>

Stacking of bar and column series should allow grouping. This could be done by making the Stacked property not a boolean, but a string to denote the group name.

This can be done by exposing a spacing property for the particular Series. 

For the time being mentioned properties can be set on the client through the chartObject of the RadHtmlChart:
JavaScript:
	<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
		<script type="text/javascript">
			function ChangeSpace() {
				var chart = $find("<%=ColumnChart.ClientID%>");
				chart._chartObject.options.series[0].spacing = 0.9;
				chart.repaint();
			}

			function ChangeGap() {
				var chart = $find("<%=ColumnChart.ClientID%>");
				chart._chartObject.options.series[0].gap = 10;
				chart.repaint();
			}
		</script>
	</telerik:RadCodeBlock>
ASPX:
		<telerik:RadButton ID="RadButton1" runat="server" AutoPostBack="false" Text="Change Space" OnClientClicked="ChangeSpace" />
		<telerik:RadButton ID="RadButton2" runat="server" AutoPostBack="false" Text="Change Gap" OnClientClicked="ChangeGap" />
		<telerik:RadHtmlChart runat="server" ID="ColumnChart" Transitions="true">
....
		</telerik:RadHtmlChart>

For the time being the property can be set on the client through the chartObject. For example:
JavaScript:
	<script>
		function pageLoad() {
			var chart = $find("<%=RadHtmlChart1.ClientID%>");
			chart._chartObject.options.series[0].highlight = {
				markers: {
					visible: true
				}
			}
			chart.repaint();
		}
	</script>
ASPX:
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="400">
			<PlotArea>
				<Series>
					<telerik:LineSeries>
						<MarkersAppearance Visible="false" />
						<SeriesItems>
							<telerik:CategorySeriesItem Y="30" />
							<telerik:CategorySeriesItem Y="10" />
							<telerik:CategorySeriesItem Y="20" />
							<telerik:CategorySeriesItem Y="15" />
						</SeriesItems>
					</telerik:LineSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>

Currently the appearance of the Series border cannot be controlled. Add the ability to change its width, color and dashtype.

Declined with the following reason: Item is duplicate of http://feedback.telerik.com/Project/108/Feedback/Details/50612

Currently the default pivot point for rotating the labels on the XAxis is the center of the label itself. Add the ability to change the pivot point to be the first symbol of the label.
For the time being the property can be set via the kendo object (http://docs.telerik.com/kendo-ui/api/javascript/dataviz/ui/chart#configuration-categoryAxis.labels.rotation.align):
		<script>
			function load(chart) {
				var align = "end"; //"center"
					chart.get_kendoWidget().setOptions({ categoryAxis: { labels: { rotation: { angle:45, align: align } } } });
			}
		</script>
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="400">
			<ClientEvents OnLoad="load" />
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Series 1">
						<Appearance></Appearance>
						<SeriesItems>
							<telerik:CategorySeriesItem Y="30" />
							<telerik:CategorySeriesItem Y="10" />
							<telerik:CategorySeriesItem Y="20" />
						</SeriesItems>
					</telerik:ColumnSeries>
				</Series>
				<XAxis>
					<LabelsAppearance></LabelsAppearance>
					<Items>
						<telerik:AxisItem LabelText="Item 1" />
						<telerik:AxisItem LabelText="Item 2" />
						<telerik:AxisItem LabelText="Item 3" />
					</Items>
				</XAxis>
			</PlotArea>
		</telerik:RadHtmlChart>

As of R1 2017 you can use the stackTotal property of the labels:
https://docs.telerik.com/kendo-ui/controls/charts/how-to/various/show-stack-total

---
old information

For the time being this can be achieved by creating an additional bar/column series with the following settings:

-series items are set with insignificantly small values (e.g. 0.001)

-the name property of the series is not specified, so that it is not visible in the legend

-the color of the series matches the color of the plot area, so that the series items and the legend of the series are not visible

-clienttemplates for the labels of this series is used in order to sum up the grand total and display it

An example on the described scenario can be found in the http://www.telerik.com/support/kb/aspnet-ajax/chart-%28html5%29/details/displaying-grand-total-in-stacked-series-labels-and-pie-series-legend KB article

Currently BarSeries/ColumnSeries are rendered like a "glass" gradient and PieSeries/DonutSeries like a "roundedBevel" gradient. Expose an OverlayGradient property that lets you control the gradient (e.g. "glass", "roundedBevel", "sharpBevel", "none"). For the time being the properties can be set through the chartObject:
JavaScript:
	<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
		<script>
			function pageLoad() {
				var chart1 = $find("<%=ColumnChart1.ClientID%>");
				//Set glass gradient to first series
				chart1._chartObject.options.series[0].overlay = { gradient: "glass" };
				//Remove gradient from second series
				chart1._chartObject.options.series[1].overlay = { gradient: "none" };
				chart1.repaint();

				var chart2 = $find("<%=PieChart1.ClientID%>");
				//Set sharpBevel gradient to pie series
				chart2._chartObject.options.series[0].overlay = { gradient: "sharpBevel" };
				chart2.repaint();
			}
		</script>
	</telerik:RadCodeBlock>
ASPX:
		<telerik:RadHtmlChart runat="server" ID="ColumnChart1" Transitions="true">
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Series with Gradient">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="15000" />
							<telerik:CategorySeriesItem Y="23000" />
							<telerik:CategorySeriesItem Y="10000" />
							<telerik:CategorySeriesItem Y="16000" />
						</SeriesItems>
					</telerik:ColumnSeries>
					<telerik:ColumnSeries Name="Series with no Gradient">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="35000" />
							<telerik:CategorySeriesItem Y="10000" />
							<telerik:CategorySeriesItem Y="20000" />
							<telerik:CategorySeriesItem Y="17000" />
						</SeriesItems>
					</telerik:ColumnSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>
		<telerik:RadHtmlChart runat="server" ID="PieChart1" Transitions="true" Width="450px" Height="450px">
			<PlotArea>
				<Series>
					<telerik:PieSeries StartAngle="90">
						<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="Pie with Gradient">
			</ChartTitle>
		</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>

For the time being series can be stacked through the chartObject. For example:
JavaScript:
	<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
		<script>
			function pageLoad() {
				var chart = $find("<%=AreaChart.ClientID%>");
				chart._chartObject.options.seriesDefaults.stack = true;
				chart.repaint();
			}
		</script>
	</telerik:RadCodeBlock>
ASPX:
	<telerik:RadHtmlChart runat="server" ID="AreaChart" Width="800" Height="500" Transitions="true">
			<PlotArea>
				<Series>
					<telerik:AreaSeries Name="Sales" MissingValues="Gap">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="400" />
							<telerik:CategorySeriesItem Y="500" />
							<telerik:CategorySeriesItem Y="720" />
							<telerik:CategorySeriesItem Y="1300" />
							<telerik:CategorySeriesItem Y="450" />
							<telerik:CategorySeriesItem Y="800" />
							<telerik:CategorySeriesItem Y="900" />
						</SeriesItems>
					</telerik:AreaSeries>
					<telerik:AreaSeries Name="Expenses" MissingValues="Gap">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="700" />
							<telerik:CategorySeriesItem Y="900" />
							<telerik:CategorySeriesItem Y="420" />
							<telerik:CategorySeriesItem Y="1100" />
							<telerik:CategorySeriesItem Y="650" />
							<telerik:CategorySeriesItem Y="600" />
							<telerik:CategorySeriesItem Y="700" />
						</SeriesItems>
					</telerik:AreaSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>

For example by exposing an additional width property.

Currently there are only three properties exposed by the MarkersAppearance - MarkersType, BackgroundColor and Visible in AreaSeries, LineSeries, ScatterSeries and ScatterLineSeries. Expose more properties like Size, BorderColor and BorderWidth for the markers in the mentioned Series. For the time being the following workaround can be used:

        function pageLoad() {
            var circle = document.getElementsByTagName("circle");
            for (var i = 0; i < circle.length; i++) {
                //Modify the size of the marker
                circle[i].r.baseVal.value = 15;
            }
            //Modify the width of the marker border
            $telerik.$('circle').attr('stroke-width', 13);
        }

Similar to the SplineArea in RadChart: 
http://www.telerik.com/help/aspnet-ajax/chart-types-spline-area.html

Currently labels for SeriesItems having Y of 0 are not displayed in PieSeries and DonutSeries due to visual considerations. Improve the handling of such labels.

Your html charts are coming along beautifully - Just want more! Particularly a Spline chart - ideally to add as option for combination charts.  This could also work as a Pareto chart.

Currently LineSeries, ScatterSeries and ScatterLineSeries expose MarkersType property that gets only three values - Circle, Square and Triangle. Add the ability to set  custom images as markers as well. 

For the time being the series can be stacked through the chartObject. For example:
	<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
		<script>
			function pageLoad() {
				var chart = $find("<%=LineChart.ClientID%>");
				chart._chartObject.options.seriesDefaults.stack = true;
				chart.repaint();
			}
		</script>
	</telerik:RadCodeBlock>
<telerik:RadHtmlChart runat="server" ID="LineChart" Width="800" Height="500" Transitions="true">
			<PlotArea>
				<Series>
					<telerik:LineSeries Name="Sales" MissingValues="Gap">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="400" />
							<telerik:CategorySeriesItem Y="500" />
							<telerik:CategorySeriesItem Y="720" />
							<telerik:CategorySeriesItem Y="1300" />
							<telerik:CategorySeriesItem Y="450" />
							<telerik:CategorySeriesItem Y="800" />
							<telerik:CategorySeriesItem Y="900" />
						</SeriesItems>
					</telerik:LineSeries>
					<telerik:LineSeries Name="Expenses" MissingValues="Gap">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="700" />
							<telerik:CategorySeriesItem Y="900" />
							<telerik:CategorySeriesItem Y="420" />
							<telerik:CategorySeriesItem Y="1100" />
							<telerik:CategorySeriesItem Y="650" />
							<telerik:CategorySeriesItem Y="600" />
							<telerik:CategorySeriesItem Y="700" />
						</SeriesItems>
					</telerik:LineSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>