Like, I'm guessing, many people here, I used to participate in Slashdot, but switched that finite collection of attention cycles to HN. I think the main reason was that moderation simply failed to bury the bad stuff and surface the good stuff, mainly due to moderator gullibility: commenters who wrote in an authoritative voice were routinely highly scored, even when insanely wrong. That and way too much simple immaturity, which, so far, seems to be successfully discouraged here.