There's always some spill-over of different topics, like programming discussion in a digital art forum. But if I want to talk about pixel art, I'll go to Pixelation, not the DeviantART pixel art subsection or some such nonsense.
I also believe small focused forums lead to a better sense of community and comradery. It's like living in a small town, where you notice the happenings of everyone, rather than living in a megalopolis where you're just an ant.