With Vista I found UAC VERY annoying. Sometimes I'd get one warning from UAC, click OK, then get another one for the same program. This seems to have been ironed out in Windows 7. It's still there, but it's less annoying.
What WAS annoying is that the box I'm testing Windows 7 on has an old Dell CRT attached to it. Windows 7 got the screen refresh rate wrong (75 when it should have been 60) and screwed up the display from 3/4 into the install process till I was able to get into settings and change it. To be fair, Ubuntu on NVidia restricted drivers does the same damned thing.
Second annoyance was sleep mode. With my aging monitor (or maybe its the video card) coming back from sleep mode corrupts the display, and cannot be fixed short of a restart. In the default configuration, the computer goes into sleep mode after 30 minutes. Easily enough fixed, but still I didn't like it.
Another problem I found, was I found it hard to locate some things in the control panel. It's different than XP.
The last issue was a driver problem, that computer's onboard sound didn't even have a Vista driver. Fortunately I was able to get the XP driver to work.
I have yet to find any other real problems with it at this point. All in all MS seems to have learned from a lot of Vista's mistakes and made improvements. I'm not sure I'll buy a copy at this point, but I'm not ruling it out either.