XP -> Vista -> Windows 7
My most prominent problems with VISTA:
1. Within the entire company where I find my employ, we have not one single running copy of Vista. The reason for this is there are several essential legacy applicatoins we use that run better on linux under Wine then they do in Native Vista. Vista has dropped a large portion of reverse compatability for what? shiny new buttons? a 3% increase in ui streamlining? I'll admit I'm one of "those people" who prefer GUI to Console, but not at the expense of functionality. Bottom line: They need to place higher priority on software compatability then they do to the application's "Prettyness"
2. Bloatware. Microsoft has made themselves quite clear, short of actual admission, that they care far more about cool gadgets in their software, then they do about reliability, security, stability, compatability, or even simple usefulness. They spent so much time improving UI gadgets in windows XP. Anyone remember the Mojave experiment? How much would you like to bet they installed Vista on the "test machine" and that's it? no internet, no other applications that were doomed to incompatability and failure. Just what's included / built in with Vista. If you run that alone, it looks pretty nice. But it is meant to be an operating system. Not "everything you will ever need". "Forget about buying games, or software, Windows Vista is now everything you wil ever need! What's that billy? you want to play Doom? that's an old game billy, it doesn't work anymore! What do you mean it works fine on your PC under XP?"
3. I know most of you have been looking at this as the worst feature: DRM. Adding DRM into an OS is a major invasion. That is, if it is forced. If it can be disabled, removed, or opted-out, then I think it is in fact a great feature, should one choose to use it. But to force this on the end user is far more immoral then the piracy that gave it reason to be. You don't put out a candle with a flame thrower, no matter how cool it would look.
4. Vista has placed far too much priority on adding new features, and far too little on fixing old ones. This has been the attitude of microsoft for a long time. This is their greatest failing  and requires dramatic change before they will be taken seriously by more then the naive few who buy anything that's new. Though a few copies may be sold to the paranoid who mortally fear falling behind, only to be beheaded by the new.
I apologise for the oversized post, I just really was both offended and amused by microsoft's constant ignorance, in their blind drive towards their goal, they burn too many bridges, and Destroy their very means of acheiving said goal. You don't take a road trip and then - in the spirit of getting their faster - start making speed holes in the hood of your car with a pick axe. If that sounds like a detached analogy, you could be right, But I feel that's what they are doing.
 Doesn't mean it runs under wine, it just get's father in the init stage. much farther.
 And yes I know full well that stand alone classic doom can take some coaxing to get running under XP. But at least it can be done, plus with Windoom, Jdoom, and other variants you can get it working nicely on XP. How many of those run on Vista?
 Especially - but not only - the warez / music / movie priates.
 And I admit, it sound fun, until you burn down the house.
 In my opinion / beleif.