@shilolo: I’m a new AB import, and my general experience so far is that you’re right—the quality of discussion is higher. I think the aggressive moderation and question limits are part of it, but I see something else here that distinguishes it from Answerbag: there’s really only one thread of discussion per question. We’re used to having multiple answers, each with it’s own comment thread. While anybody can participate in any thread, the diversity of the discussion tree means there’s more ‘privacy’ on each comment thread—your post is less likely to be read by a lot of people.
By contrast, everybody on the thread here gets exposed to what everyone else is saying on the topic—people see that, and “rise to the occasion” more. There’s more of a sense that if you say stupid stuff, you’ll have a large audience for it. I think that provides extra incentive for people to think a bit harder before posting—something Answerbag users need badly.