Don Park makes an interesting observation on the average computer user, and Dave Winer agrees.
I've rambled about this topic quite a while ago: 95% of the UI improvements in Windows Vista or MacOS X are irrelevant to 95% of the computer users out there. Why? Because they don't address the fundamental challenges these 95% face.
I would even go farther and say that 99% of all past improvements to the Windows Explorer or Macintosh Finder (remember the spatial orientation discussion?) were irrelevant to 99% of the users.
[Update: And I don't subscribe to the theory that the Web is a fundamental advance for these 95% of the users, as David Berlind notes. It's not that the desktop (OS) is flawed and the Web-based UIs change that. Web-based UIs just have different kinds of fundamental problems.]