What is the actual difference between a site like StackOverflow (or their sister sites on the exchange) vs. a forum with a question-and-answer functionality built in?
At its core, as Ben alluded to, each question is essentially a "topic/thread", with immediate replies considered "answers", and further sub replies considered "comments".
An accepted answer needn't federate, though it can always provide that information via a separate ActivityStreams property.
My assertion isn't that StackOverflow does anything different "technically", but that their network effect and centralization, along with being the only good option to ExpertsExchange, allowed them to prosper.