In a way reddit is the ultimate model. When the main room gets to big you can go and make a new room (subreddits), but still in the same house where everyone else is. Brilliant model in my opinion and one that I believe will be followed by successful future discussion board systems in years to come.
An in-between option is the http://lobste.rs model of having tags on stories, and letting users filter on tags. Allows me to ignore a few topics I just don't care about, without really splitting the community (some of the community opts-out of a few topics here and there, but it's by and large one community).