I think LJ killed forums by being so much less hierarchial. Post counts and member statuses are very intimidating. And it just works better for networking and meeting people - clicking on the person's name takes you to their personal journal, which looks different from everyone else's, and gives you a sense of who they are. Friending and reading an flist is something you can't do on forums. It's MUCH easier to post on a post by someone you don't know than to jump into a discussion. LJ comments are made for getting multiple replies to one comment and being able to have several discussions at once with different comment threads - and you don't have to quote things to reply to comments, even when other people have replied before you.

Also, signatures are really annoying.