I've been using Ubuntu (Feisty) solidly for about 3 weeks now. It's /ok/ but there are lots of things wrong with it. I'll probably move back to windows at some point.
Anyway, here's what I found:
Touchpad behaviour is just horrible. I had to plug in a usb mouse.
Fonts are horrible, they always have been.
Most of the default themes look like they are from the early 90s.
Thunderbird's spam filter SUCKS. With Outlook I hardly have to train it, it just works. Thunderbird I've been training for ages and it still can't get it right.
Video streaming support is poor - I've had firefox crash (by crash I mean spontaneously terminate with no errors or dialogs) numerous times when trying to view a video online. Half the time the media player just doesn't do anything.
Hardware drivers are still an issue. It took me a long time to get my wireless card working.
It can't turn off the backlight on my laptop's lcd.
Various other problems. I just don't think Linux or Ubuntu have had enough time to mature for wide spread desktop use. There are too many usability issues and stupid bugs that have to be fixed. This is all in my opinion of course.