I've had occasion to give this one a lot of thought, both as a forum member and admin. LJ gives people a community they can control, rather than being subject to someone else's rules and the whim of the moderators and admins. I think that's one of the biggest draws of LJ. Second, novelty. Blogging is still relatively new, and I still remember when it first showed up in one of my forums. Everyone rushed out to get one. It's a networking tool that allows you to reach more people. You usually have to seek out a forum for a certain interest; in LJ you get friends-of-friends-of-friends stumbling across your blog.

I still prefer forums, though. I find the range of topics FAR more diverse, and it's a lot more organized than LJ. I could find the subject I'm looking for in a forum a lot faster than here. I also find it fosters a greater sense of community. Even LJ communities often have pretty strict posting rules; I'm a member of a few where the mod deletes anything that goes off topic. I know the members of my forum far better than I know any of the members of my LJ communities.

I think the pendulum will swing back the other way eventually. Forums are going to have to work harder to earn their members--I speak from experience there--and LJ's going to have to counter, but I don't think forums are going to go the way of the dodo.