Couldn't Windows 7 Be Backported To XP? (sorry about the useless link)
http://www.microsoft.com/windows/windows-7/what-is-windows-7.aspx
Some features, such as the security changes for autoplay on flash drives due to the Conficker worm to XP and the ribbon interface for Vista, have already been backported. Windows 7's Windows Update is also part of Vista SP2 now. One feature that isn't non-trivial is branchcache. My point being that all these new features are all user-facing, non-system features that could be part of XP and Vista. But the HN community probably already knew this.
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They could probably have implemented Aero Snap on Windows 95. And I'm sure the shareware authors will backport this sooner or later. Same with most of the changes to the task bar.
On the other hand, some of the major graphical changes are probably more complex than they seem, and rely on significant changes to the guts of the OS.