Features to support:
- Text blocks with basic formatting, preferably draggable, and preferably with customizable borders/backgrounds.
- Basic drawing functionality such as pencil, brush, lines, circles, rectangles etc. Of course with color selection for lines and fill. 
- Line thickness selection. 
- Being able to move the objects around the board would be nice.
- Connecting objects with some sort of connector lines, pretty much like the map structure in SketchFlow, if you are familiar with that. 
- Some basic shapes, such as arrows and icons that can be dragged/dropped on to the board.
- Support for uploading images and dragging them onto the stage. 
- XAML export (necessary for saving a drawing)
- Printing
- Magic marker/highlight functionality
- Collaboration support. This is a big one, and I am not sure how it should be implemented. Full collaboration functionality out of the box would require a service running duplex WCF or something similar. Alternatively, perhaps Telerik could provide us customers with an API and some sample code. Then we could handle the back-end stuff ourselves.

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Our XAML team has recently reviewed the status of this feature request. Since most of this requirements are covered by our RadDiagram Control / Framework we are changing the status to Completed.
Please follow RadDiagram documentation:
http://docs.telerik.com/devtools/wpf/controls/raddiagram/overview.html