can't compete with "good enough"
Nobody cares about operating systems - they don't actually do the work you want done. That's done by the applications. The best thing an operating system can do is not crash and keep your data and working environment relatively safe.
From that basis, it's fair enough for a new PC to run Vista - provided of course that the user doesn't have to learn new stuff, and that it will support all the bits'n'bobs that person (or organisation) wants to interface it with. Oh yes, and that it runs the applications you actually want to use.
That's where the problems start, Vista gets in the way. The ideal operating system would be completely invisible: just doing it's thing quietly in the background and not getting between the users and their objectives. Even worse, as an operating system it doesn't offer any compelling new benefits (note: not features) that a large group of people actually need. This isn't Apple, so eye candy doesn't count, guys.
This is where XP was successful, not because it was particularly good, but because it's predecessor was not up to the job. The problems and unreliability of W2K - even with 4 service packs, meant that people were willing to shell-out for XP and the advantages it gave: not getting in the way, not crashing and letting them get on with their work. That it would also run their old hardware and applications without having to buy new "XP approved" versions helped a lot, too.
So far as the future goes, there are no operating systems "killer apps", the lack of backwards compatibility for both hardware and software is a major disadvantage and crushing hardware requirements are the last straw. Until the people in charge of Vista Mk2, or preferable XP Mk3, realiise that, like a good butler the O/S should only be noticeable by it's absence, and that it's neither a feature nor a benefit in itself, they will always be competing with the "good enough" older stuff.
My favourite quote from the release of Vista was from an obviously non-technical woman looking at the Aeroglass interface. When told that Vista had cost $10Bn to develop, her response was "Is that all they did?" Pretty much sums up the whole world's response.