The SitePoint vBulletin forums had a few “mega threads”. But they were not what I’d call discussions. A few were “game” threads which were more post-to-post where there was no presumption that anyone would read every post from first to last. And there was a catch-all Introductions intended to consolidate multiple “Hi, I’m new here” threads. There were also some “opinion” threads which were little more than a series of “I like __ best” with a sprinkling of substance here and there.
I guess whether or not a community wants non-discussion topics in its forum is up to the community. But in my experience and in my opinion any discussion that starts to get anywhere near “mega” has become full of tangential and repititous posts that make reading from first to last such a major task that it is highly doubtful anyone would do so.