Forums treat all threads as equal. Usually they are listed by the time of the last post. You can get any thread back to the top by "bumping" it. Within threads all posts are equal and listed by time of entry. In a long thread the only way to see the best comments is to read the whole thing. Voting mechanisms are either poorly implemented and almost useless or not available.

For many subjects a 'social news site' style of threading is probably a better fit than traditional forums. Right now everyone knows how to use a forum. Despite Digg, reddit etc. social news sites are still a niche. But I have often thought when using forums that I'd like to be able to vote up a comment or thread. Over time, a 'social news site' interface might become a widely used alternative to traditional forums.