It would help to use RadWindow like System.Window with:
- WindowInteropHelper for correct modal behavior
- Set window in task bar, also set taskbar icon/title
- Use RadWindow as MainWindow in Application (set Application.MainWindow property with RadWindow 

When a modal RadWindow is opened clicking out side of the Window (the overlay) should make a the Window blink and a sound should be played.

rendering problem is when minimized only

RadWindow.Confirm default button is OK, in order to change that presently we have to extract and modify styles, set FocusManager.FocusedElement binding to Cancel and set the ContentStyle property in code. 
It would be much more user friendly if this could be controlled by a simple property.

Available in the 2016 Q1 SP1 Release.

Make it possible to be able to drag the Window not only from its Header. In some cases it is useful to be able to drag the RadWindow from within its content.

When the attached TextOptions.TextFormattingMode="Display" property is set to the RadWindow the window stops auto resizing its size according to the size of its content.

In a scenario where a modal RadWindow is closed and it is opened via a separate thread, the Host property of the WindowWithNoChromeWindowHost class in the BringIntoFront method may be null, which raises an NRE exception.

Currently the template of the dialogs should be customized:
http://www.telerik.com/community/code-library/wpf/window/how-to-modify-the-alert-dialog-icon.aspx

Available in the 2016 Q1 Release: Added an IconTemplate property that provides an easier way to change the predefined dialogs' exclamation/question mark images.

If a ContentControl is placed inside the control and the ResizeMode is set to NoResize the SizeToContent doesn't take effect.

As a workaround all you need to do is set the ResizeMode to NoResize in the Loaded event of the ContentControl.

The property is present, but it is not implemented. 

Available in the 2015 Q3 Release.

Available with the R3 2016 SP1 release. 

This happens when RadWindowInteropHelper.AllowTransparency is set to False and RadWindow is implemented as UserControl. The Window need to be maximized firstly on the main screen and then on the secondary in order for issue to be reproduced.

The issue also affects the ToolWindow that RadDocking uses for its floating Pane. In order to reproduce the problem, the primary monitor needs to have its DPI higher than the secondary one.

The Window doesn't resize correctly when dragged from the primary monitor (with taskbar) and docked to the top of the screen of a secondary monitor (no taskbar) in order to maximize it and "Show window contents while dragging" Windows system setting is disabled.

The header buttons elements are not on the proper places when the window is on a monitor with different DPI. To reproduce this the DPI of the monitors should be different for example 125% (main) and 150% (secondary). Also, to reproduce this you will need to enable the DPI awareness of the application by setting the dpiAware and dpiAwareness settings in the app.manifest file.

Maximizing RadWindow from the header button shows a window with the size of the content if SizeToContent = true. If you maximize with double click on header it is ok.