Sort of echoing what others have said, forums provide a type of user interaction that isn't really supported very well on other platforms. Forums were initially created to foster discussion between people, potentially long-form, which is easy to follow and fully inclusive (meaning it doesn't give priority to the OP or anyone else).
Other sites don't really have these qualities, since they were made to support different kinds of interaction. Twitter was created for short updates. Blogs were made for people to write short responses to a single instance of monopolized, long-form content. For all that it's used for discussion, Reddit is ultimately optimized as a content-aggregator, which makes discussions in comment sections hard to follow sometimes since it works like a tree and you can only really reply to one person at a time.
Forums, on the other hand, are optimized for what I think the rationality community ultimately needs to sustain itself: egalitarian discussion. My only concern is that, currently, the community is pretty much run by a few "whale" contributors (Eliezer, Scott, Ozy, etc.), which would seem to imply that the community doesn't really want to have open discussions like this. That may just be because we haven't been using platforms which support this sort of thing - I dunno, I guess we'll see.
Plus, I mean, how can you not love phpBB forums? I practically grew up on these things.
Discussion for all topics (until the forum becomes large enough to justify splitting things up this will be where all topics go)