But Stack Overflow has a consistent problem with interesting questions getting deleted by fascist moderators, and it seems to only be good for Questions and Answers. It's not a place to pontificate about fibromyalgia, for instance.

That's not a bug, that's a feature. SO was designed and restricted for a Q&A format; it deliberately omits features like comment trees. Its creators believe that it succeeded because it was restricted, since a more specific problem is easier to solve. They've gone on to try and solve the more general discussion forum "problem" with Discourse, but it's still very lean on features - because it's explicitly designed for mobile touch interfaces and for reading over writing.

Personally, I'm not convinced that SO - as a community - couldn't have succeeded just as well, or better, with quite different software features. But that's what they believe.