While I like Rimesume's idea I would even try to push it a full step further and suggest that more popular subject specific journals just be made into actual forums. They would still take up the same server space anyways right?
For example
Vataro's journal is the unofficial Anime forum. Now as much as I love Vataro, being a forum that only 8-10 people know about does hurt discussion. It has only been up a short time and already has three pages (compared with the Fantasy and Scifi forum that has been up forever with 4 pages) and is only known to a few people. If it were made a listed forum I believe traffic would significantly increase (nerdy MMO gamers and anime geeks are an often overlapping group); I highly doubt it would be any less popular than scifi fantasy forum. I believe the site would benefit form the increase in traffic not only generated from ad revenue, but because anime geeks are loose spenders who would easily buy premium just to have a scene/character displayed in their avatar.
Darkenbuddy's automotive forum has received a positive resposne as well (though it is still very new).
For a site as large as Allakhazam to only have six non MMO related forums is rather unusual. OOT usually gets lumped full of related threads that could very easily fill a separate forum of their own.