The main thing they got right is that pages on the Stack Exchange network include (as primary content) exactly one question, followed by the answers to that question. That's it. All other side discussion, requests for more info, expressions of gratitude, etc, are secondarily attached as comments. They are visually and conceptually separate.

In retrospect, this is totally, obviously what you want, but there was a lot of momentum behind making Q&A sites look like a forum.

In other words, their success is well-deserved.