The best online forums are the ones devoted to a topic and run by people who believe in what they're doing. HN, in fact, fits this description to a t. bluelight.org, which I used to moderate, is a great example of how a dedicated community can generate productive discussion even when most of the members are heavy drug users without college degrees. Quora, by contrast, has some of the smartest people from all over the world, but it's hardly a "community" and is susceptible to its own forms of groupthink and political squabbling.

If you're specifically looking for discussion on politics, philosophy, and religion, aka the three things that end up on every forum that isn't about them, I suggest reading books and peer-reviewed journals.