The best-quality Buffy and Angel talk I’ve found is at the Television Without Pity forums. The boards are tightly moderated and the threads tend to be long and intimidating for newbies, but as a new Buffy fan myself, it has been a great place to turn to for insightful fan commentaries and smart discussions. I think of TWoP as the television-focused equivalent of the Straight Dope, with lots of funny, smart, snarky, educated posters.
TWoP’s Buffy boards: http://forums.televisionwithoutpity.com/index.php?showforum=599
TWoP’s Angel boards: http://forums.televisionwithoutpity.com/index.php?showforum=701
As for comic book talk, I find Marvel tends to attract younger and more immature fans than DC for the most part, so good luck. But I find Millarworld (run by comic writer Mark Millar) tends to have good discussion, and moderators keep things from getting too fanboyish and sophomoric. Industry pros stop in there to post and interact with fans all the time:
Writer Geoff Johns also has good, well-moderated boards, but they are much more DC-centric: http://www.comicbloc.com/forums/index.php?