Support SmoothStreaming in WPF
Silverlight SmoothStreaming is provides ability to display high definition video in Silverlight applications. Why we don't have the same ability in WPF? This would open possibility to implement rich media center applications (for example running on TV set-top-boxes).
WPF isn't planning to target set top boxes and similar. That's Silverlight's space.
The main reason Smooth Streaming isn't in WPF is that the media stacks are very different. In addition, Silverlight relies on the Media Stream Source API to implement smooth streaming. Net result is implementing IIS SS would be a fairly large chunk of work if we do it.