Vista DWM
I just found this great blog post by Greg Schechter, about the Desktop Window Manager in Vista. Read it if you haven't.
One of the key bits is that really there's no such thing as external window invalidation anymore. When you move a window and reveal a portion of the window behind it, the application that owns the newly-revaled window doesn't have to wake up and repaint it - Windows has a cached copy of the entire window already.
It doesn't seem like a big feature, but man that must have been a lot of plumbing to rewrite.