Well, XP is based on Windows 2000 codebase which was in development for 5 years. So basically XP is based on a very old code and it required a lot of patchwork after the release.
But Windows Vienna/Blackcomb and Longhorn was a different design but it looks like the features of vienna was very extensive or futuristic to be included into the vista release, that's why they stripped some of the features in Longhorn (like WinFS, Virtual Folders) to match the release date. And now MS is hinting that Windows 7 is all about user interface enhancments.
I found some concept videos: (not real)
http://www.youtube.com/watch?v=RsJC2vzqsWg
http://www.youtube.com/watch?v=1YwCev1p1U4
BlackComb/Vienna: http://www.youtube.com/watch?v=OgEEb0G56Js
But Windows Vienna/Blackcomb and Longhorn was a different design but it looks like the features of vienna was very extensive or futuristic to be included into the vista release, that's why they stripped some of the features in Longhorn (like WinFS, Virtual Folders) to match the release date. And now MS is hinting that Windows 7 is all about user interface enhancments.
I found some concept videos: (not real)
http://www.youtube.com/watch?v=RsJC2vzqsWg
http://www.youtube.com/watch?v=1YwCev1p1U4
BlackComb/Vienna: http://www.youtube.com/watch?v=OgEEb0G56Js