I suspect communities have a natural life cycle and most are doomed. Either they change unrecognisably or they die. This is because the community members themselves change with time and change what they want, and what they want and will put up with from newbies, and so on. (I don't have a fully worked-out theory yet, but I can see the shape of it in my head. I'd be amazed if someone hasn't written it up.)

What this theory suggests: if the forum has a purpose beyond just existence (as this one does), then it needs to reproduce. The Center for Modern Rationality is just the start. Lots of people starting a rationality blog might help, for example. Other ideas?