Scrolling will sometimes stop working in some applications and might resume working or not. Vista is just full of surprises!
I hear sleep mode has been overhauled in Vista and I can attest to this because when my computer goes to sleep it is impossible to wake it up again!
I can't switch user reliably because somtimes it just freeze.
Aero, the fancy windowing effects, will sometimes disable themselves for some time until I close the arbitrary application that caused it to disable. I have not yet figured out a pattern for when or why this happens.
The user protection thingy is getting quite annoying. Not when it asks me to verify starting a program for the first time, I like that. But why does it ask me to start the same program "for the first time" AGAIN AND AGAIN AND AGAIN!??!? And forget about using WinRar, if you try to extract something using the context menu it'll just fail silently doing nothing. I suspect this is Vistas fault, not asking me for confirmation or whatever. I don't know why I need administrator access to extract an archive, but I'm sure Micrsoft Knows Best.
Setting a program as the default for an operation rarely works. It was hard to get uTorrent to stick as the default for torrents, but somehow it suddenly worked after two weeks. I dunno how I fixed it, but I don't want to infuriate the Vista God lest it smite this unbeliever.
Last, but DEFINATELY NOT LEAST, windows explorer (the file manager, not the browser) keeps crashing every hour or so!!! WHAT THE HELL IS GOING ON!?
Of positive note, there is a lot more than a fancy window effect to Vista, it is a lot more helpful for starters. Where XP previously only would say "Nah, you can't do that", Vista will say "You can't do that before you've activated XYZ. Click here to activate XYZ and proceed.".
Crazy as I am though, I'd still prefer a stable and working system to one that is slightly more helpful to you. Besides, my experience is that Ubuntu is both stable and helpful, so unless you are using some software that only works in Windows you should give Ubuntu a try. Using a Live CD you can even try it out without installing it.