11D: Dangerous Women: Unfogged is a tough crowd, but I like'em. They're smart and witty, and there's real back and forth. Plus people are willing to stick around for hundreds of comments because interesting things happen even deep in the threads. The only other place I visit regularly where that happens is Making Light...
You will like the community of commenters and webloggers at http://www.unfogged.com/ if that is the kind of thing you like. It and Making Light http://www.nielsenhayden.com/makinglight/ are, in my experience, unique in the intelligence and wit of their participants--Algonquin round tables for the onrushing Age of the Global Village. But Unfogged, especially, will not be to everyone's taste.
Why are there so few gold-standard high-quality internet intellectual communities around? And are there others of equally high and consistent quality of which I am unaware?