I joined SA in '03. A few years later when Reddit launched, I rejected the "hierarchical" nature of Reddit posts, multiple posters having their own conversations in different parts of the same post just seemed harder to keep track of, similar to an email chain with new people joining and replying to different parts of the chain. SA's threads were, and still are, non-hierarchical.

At some point something interesting started happening, at least in the sub-forums I participate in. So called "megathreads" started gobbling up what would have been new threads, e.g. the "Python questions that don't deserve their own thread" thread. It was (and still is) enough to simply bookmark individual megathreads of interest, rather than the sub-forum itself. It was as if the megathread itself had become hyper-specific forums in their own right.

I see megathreads now as slow-moving asynchronous chat rooms, with a good membership mixture of regulars and newcomers. The pace agrees with me.