If you're like me, you tend to be impressed by the small things. Mac OS X has Expose and live updates in the dock, Vista has the Flip and live updates too in the start menu and alt-tab. Vista's ability to do this is build into the Desktop Window Manager and actually can be accessed through API.
Sven Groot has created an example of DWM over at Channel 9 and shows you how to do thumbnails in it. Scott Hanselman shows a few applications that run off of DWM too. My Expose even has the source available.