A StackOverflowException occurs when you press a RadDropDownButton placed into the DropDownContent of another RadDropButton.

To work this around, subscribe for the LostMouseCapture event of the parent drop down button and handle it.

private void DropDownButton_LostMouseCapture(object sender, MouseEventArgs e)
{
      e.Handled = true;
}

The native radio button supports validation. Implement validation for the RadRadioButton

The current workaround we can offer is to define the following style for the RadioButton element:

			<Style TargetType="RadioButton" BasedOn="{StaticResource RadioButtonStyle}">
				<Style.Triggers>
					<Trigger Property="IsChecked" Value="True">
						<Setter Property="BorderBrush" Value="{telerik1:FluentResource ResourceKey=AccentBrush}" />
					</Trigger>
				</Style.Triggers>
			</Style>

Where 
        xmlns:telerik1="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls"

Custom property values for BorderThickness and BorderBrush are replaced after button click.

To reproduce, use the following code, you'll see that a highlighted border appears after clicking the button.

<telerik:RadButton Background="Gray"
                           Foreground="White"
                           BorderThickness="0"
                           BorderBrush="Transparent"
                           Content="click me"
                           HorizontalAlignment="Center"
                           VerticalAlignment="Center" />

Reason for declination: 
The border that you see is the focused state of RadButton, which is a separate visual and its BorderThickness is not template-bound to the BorderThickness of the control intentionally. We believe that a control should have focus even if its default border is removed. In our latest themes we are trying to keep this behavior. In the Office2016 theme there is an exposed palette property, which you can use in code-behind if you want to remove the focus: 
Office2016Palette.Palette.FocusThickness = new Thickness(0);

The UI freezes if the DropDownContent of RadDropDownButton has a native Run element and the CloseOnPopupMouseLeftButtonUp property is set to True.

If you have a tooltip on a button and focus the button before hovering it, the tooltip is not shown

Implement mechanism that allows positioning the drop down content of RadDropDownButton when the DropDownPlacement property is set to Absolute or Relative. Something like DropDownHorizontalOffset and DropDownVerticalOffset.

MouseLeave event is never fired after we change the Them of the controls from a control located in the DropDownContent. Therefor the controls IsMouseOver property is true and looks like it is hovered.

Arg_COMException in some cases when used with RoutedUICommand

Update the Buttons AutomationPeer to use the AutomationProperties attached properties.

As a workaround you can create a custom button control deriving from RadButton. That would allow you to create a custom AutomationPeer class that can return the AutmationProperties.Name value to a narrator application

*  3 Image properties (normal, hover, disabled)
 *  showBorder/flat true/false (Essentially making only the image appear as a button)
 *  if the 3 images are not set then the visual state manager should modify the image to appear disabled or hover with a highlight / mask overlay (or whatever the proper term is).

When a RadMenu/RadContextMenu is used as DropDownContent of a SplitButton/DropDownButton, the DropDownContent  cannot be closed after sub menu items are displayed

Should work the same as the RadContextMenu. 
Also add support for setting effect (DropShadow... ) to the Dropdown itself.