The other sites I tend to go on, discussion-wise:
Aven - the Asexuality Visibility and Education Network - discovered this when I was having a bit of an identity crisis a few years back, I worked out in the end I'm not asexual after all, but I still hang around the site because I find the subject matter interesting, you get some really unusual/intelligent debates on there as well, mostly centring around relationships/sexuality etc, but quite different to other sites I've found.
B3ta - haven't been on here in a while but as someone above said, the question of the week section attracts some extremely funny stories, touching ones, and then a load of puns, toilet humour and sick jokes. If you can look past those the rest are worth it, but I got fed up of the immature stuff so am giving it a miss for a bit.
Mark Reads/Mark Watches - this is utterly brilliant, just some random guy called Mark who reads through various books and watches series, one episode or chapter at a time and reviews each one by one. He's very funny, very aware of various issues, and usually has a really interesting take on things. They have a fantastic policy in that they will allow swearing, discussion on almost any subject, but things like ableist, sexist, any kind of discriminatory humour are not allowed. Rape jokes are immediately deleted, for example. Somehow the discussions can range from discussing a character's clothing in one episode, to heavy subjects like feminism, politics or religion.
MoneySavingExpert - again used to go here a lot. I drifted away from it when I found MN though. The money-saving advice is good and the sense of community on certain boards is nice, but I find the conversation in general more intellectual here (even though that sounds awful and wanky)
Simbology - specialist forum for the game "The Sims 2". Probably of little interest to most people.