I post on /a/ and /mu/ mostly. /a/ is a little bit like old /b/ sometimes, since it gets a lot of threads that aren't really about anime or manga. It has a lot of shit though, inevitably, given it's one of the biggest boards and all.
/mu/ just sucks, I don't even know why I post there. The best thing you can say it about it is at least it hates all music equally. (Although to be serious, the rare thread that is a) filled with people who clearly know a lot about music, and b) not reposted every day, makes it all worth it.)
The rule on 4chan is basically the more popular the board, the more resembalance it has to /b/. Especially as /b/ is fairly well known now, people come there first, then spread to the other boards and think they're the same.
A lot of people don't seem to believe /b/ was ever different, but it was. Once you knew that most Anons were just acting retarded, now you know that... well... most Anons are retarded.
4chan would've been better off if it had never become so big, honestly.