For the book-related discussionsthat so many thrived on, Cheryl's new forum, A Meeting of Minds, seems to be the best match. It hasn't really engaged me, though. I suspect that's because I popped into the book forums so infrequently. There's only so many times I want to read a theory/idea described in yet a different way. I've maybe had one post there so far.
For the 'fluff', as in the PCX, Fiend-stuff, and most of the contents of the Games & Silliness Forums, and the light-hearted discussions of general things, then the AKT will suit. Don't expect a lot of sailorchk, and certainly don't look for Dawn as a host. That ought not be a surprise, since I didn't turn up in many of those discussions in their previous existence on the soon-to-die KTBB.
For fandom/writing...club fandom looks like the current best hit on shewhoguards's Pern Alliance. Anna's newly-refurbished Pern Fandom Forums has promise in the writing area, if it draws enough people, and may turn out to be my new home. The new RP forum looks inviting. I think, however, that most of my writing fix will come from the LJ communities.
And of course, for the general discussion with an edge, where an idiot's an idiot and thinking before posting is encouraged, there's still the BPOI. I might eventually turn up there...the reasons for not joining (which aren't what you’d expect...more along the lines of not wanting someone to think I was following them to harass them) are so long past...but then again, maybe not. 'Cause what's the purpose of not having to deal with a few particular full-of-themselves asshats and then go have to deal with a few particular full-of-themselves asshats? (Particularly one obsessed with trolls.)