I hate the fact that users are aloud to swear and flame against other members and against the video. They should at least censor words; kids go on YouTube everyday.
Another thing I don't like is the quality of most videos; if the video is smaller than the screen, it will resize it to an uncomfortable resolution. In my opinion, the webmaster(s) of YouTube should make the screen as big as the resolution of the video uploaded, unless it is too large. Also, the quality of the videos in 'Fullscreen Mode' is just terrible.
I also hate the buffering of the videos, but that's not much of a problem.