I think part of the problem is that the forum doesn't have a real purpose. HalfPixel was about PvP and other web comics. Yes, we talked about other things, but PvP is why we were all there originally. When Halforum started, it was born out of the outrage of HP being taken down unexpectedly and bile toward teh Kurtz. Even if that is not why you came to this message board, that is essentially why it was made - Kurtz pissed off a lot of people, some wanted to make sure they didn't lose the friends they had made, and we all wanted a place to call home and be able to discuss what had happened.
Now that's all basically settled down, except for the one or two who like to hang on to those bad feelings, and the forum seems like it has no real direction. What do we now have in common here? It's not a web comic board. It's not a video game board. It's not an art board. It's not a board where all the members are working in the same field. It's a bunch of people chatting. I think that is where we are losing new members and why we are seeing people going at each other more. Bringing in "new blood" will work for a while, but then those people don't stay new forever and the cycle continues.