As a heavy forum user for every game I've played, I'm actually saddened by the fact that I don't really use these boards. Why? I have no reason to come here. General discussion is littered with complaints at an extremely high pace,(Which is normal) and all of the somewhat counter-intuitive sub-forums don't really appeal or interest me.
I'm huge on community within MMOs, but the community I, and many other people tend to care about are the ones specifically dealing with them and theirs.
I understand that you feel like server specific forums would spread you all too thin on moderation, but let's be a little realistic here... You don't need to moderate as heavy on server forums. You just don't.
Allow me to elaborate a bit. Building the server specific community means finding out who the heroes, villains, white knights, scumbags, and every other flavor of being are. This requires a more lenient stance on moderation. I rarely report people on the forums, but when I do it's because they were doing something beyond calling out, bickering, or name calling. IE trying to sell gold, or linking to keyloggers/hacks.
Let the report feature be your guide for the absolutely necessary moderation. Other than that, skim through, and let the community police itself.
I have a feeling that until you implement server specific forums, your grouped sub forums are going to be quite dead, or filled with more people demanding server-specific forums.