My biggest gripe is that Vista is a pain in the ass to install in the first place. I had to disable my NICs to keep it from trying to look for updates during the install and thereby locking up the installer. Also there are a lot of people who still have SB Live! cards, but are there properly functioning Vista drivers for that which include support for the Live! Drive front panels? Noooooo. I got it working once with the creative drivers for XP, but had to nuke the install and start over because something broke. The second time the Creative drivers didn't give me the front panel support and I had to turn to the kX project, which doesn't work quite right, but I can at least use the secondary mic input with it. Then there's the loathsome start menu and the fact that you can't even change it to XP style. It's either Vista style or Win9x/Me style, and don't even get me started on that search bar instead of a regular "Run" item... I could go on at length, but I've said enough at this point about the irritations.
Now, there are a few things they did right. Like being able to set volume levels individually for programs, but even that was done at the expense of a proper mixer. The categorized control panel is much better than XP's (I still switch to classic control panel for my own use though). And it's nice that the UI doesn't look like something from Pixar.