I think the biggest problem is that there's no reason for most people to upgrade to Vista, whereas the 95->98 and 98->XP upgrades were very useful. I can do everything that I want on XP, I don't need to upgrade my pc to run it, and my XP system is rock-solid stable. I keep a pretty trim system and I can't remember it ever crashing on me.
What can I do in Vista that I can't do in XP? Play Halo 2? See through my icons? Woopee.
I think that in a couple of years Vista will finally mature (after Service Pack 1 maybe?) and I'll upgrade. But there will have to be games or programs I want to run that won't run on XP before that'll happen.