I've probably said nearly everything I'll post here before, because if I can potentially complain about it, I probably would have in the past. But never the less, the things I do not like to see in internet forums:
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Aka 'You need javascript enabled to view this site'
Seen it a lot on vBulletin forums recently, and it's certainly not a default feature, and I hope the creator of that horrible modification or code gets forever banished from the internet for crimes against usability and website design decency.
I also don't like default anything. Any more default banners, forum icons, smilies, BB code, general icons, forum buttons or colours, and I will probably close the page never to return. Default = bad. I just hate default internet software icons and what not.
Empty forums. You know, 0 posts, 0 threads...
Private forums and login only forums. Unless there's a very good reason for it, don't bother. I also don't like it when a forum is private but you can't even tell from the forum listing. Or join to view replies. Or what not.
Excess of advertising. Pop ups are bad. As are obnoxious flash advertisements.
The username 'admin'. Really, really impersonal in my opinion, and leaves a generally bad or strict impression of board management.
Limitations to the point of insanity. Reminds me of Nsider or Gamefaqs, which is NOT a good thing (for comparison, the latter makes you have to be a member for about two months to use the search system, the latter made you have to be a member and have 1000 posts for a freaking signature)