I've had a year of games that won't run on Vista due to various incompatibilities, and probably only one game, BioShock, where I allegedly got to see anything of DirectX 10, though I couldn't tell you what part of it you couldn't see on XP.
Just recently I bought a 24" iMac (because, you know, I need a machine where I can actually get things done, where I have Unix at my fingertips, and where the UI doesn't make my eyes bleed). I decided to devote some disk space to XP through Bootcamp, which allows you to run Windows natively on an Intel Mac. The result? I can play Windows games again -- yay!
So now I have a snazzy machine running Vista and a snazzy machine running XP, and you can guess what my desk typically looks like:
Yup, the $4k Vista machine spends much of its time on a panic blue screen, while XP on the iMac hums along just fine.
Honestly from my past experience I thought "Windows always crashes" was an out -of-date myth, but just yesterday I must have restarted the Vista machine half a dozen times after it panicked, something that has never occurred on the iMac, for either XP or the Mac OS. Spybot can't find anything, and the memory tests out fine. Who knows.
Anyway, I gotta go play some more Overlord on my iMac!