I am very excited about making the source code to the .NET Framework publicly available. I think this will allow developers to debug into the .NET Framework source to help their understanding of the platform. Imagine trying to fix a view state issue and being able to literally step into the framework source code and see what is actually going on.
While I know many folks have been using .NET Reflector for this purpose, I think the debugging support from within Visual Studio 2008 is really a great addition... Plus it gives Lutz more time to work on other features ;-)
Scott has an excellent walk through and integration works and Scott Hanselman and Shawn Burke recently recorder this Podcast.