forums where all users have 800x600 sigs, with flashy Dragon Ball and whatever images, 200x200 avatars and the posts are one liners.
popup ads I think pretty much anyone hates.
mods/admins that think they are better than the users... I mean, c'mon, you are just the janitor.
boards that limit signatures too much. boards that don't limit signatures at all.
boards full of trolls, where everyone's hobby is to get to each other, where you don't make any friends, just enemies.
boards with 50 empty forums. I don't dislike them but its sad, been there done that. mine still has too many forums, but I can't bring myself to chop any one of them because they seem to make sense.
youtube mods with 1000 youtube videos. If I want a site with >1000 youtube videos, I go to youtube. now, a forum called "YouTube" where people share videos they like (using AME) is pretty cool, and is always popular.
what else... idk already complained too much
edit ===========================================
now for the forum categories I like
Anything goes
Movies and Television
Post Jokes Forum
Post Videos Forum
Interesting Images Forum
News and World Events
in the hands of a nice community, those rock