The most interesting thing I saw out of PDC was the concept of Libraries. It sounds a lot like what I was advocating Apple do in future OSX releases. The whole file/folder/tree concept has to go. It's becoming impossible to really manage a file system on such a granular level. I don't think people are quite ready for a 100% meta-data interface but that's going to be the next step.
Overall I think they're heading in the right direction with the Windows 7 UI. The big question is if they can break all the bad habits of Windows developers and really enforce a new GUI design policy that is consistent. Right now with Vista you can see the whole range of Windows UI elements from Windows 3.11 (try to install a font via control panel if you don't believe me) all the way up to the newest .NET/Windows Forums style -- plus the various proprietary controls mixed in. It's a complete mess. At some point Microsoft just has to punish developers who don't try to buy into the consistency of a unified GUI or things will never get better.