I've mulled this over in watching the community. Unfortunately (for other sites), I've come to the conclusion that it's a cultural thing and therefore subject to the whims of the dominant culture at any time.
HN certainly does things (thanks, team!) that make this more likely, such as playing around with the ability to downvote replies to yourself, etc.
But I think a larger part of it is "monkey see, monkey do to fit in." If I see an angry, fact-lite comment getting grey-bombed into oblivion, I'm going to probably be less likely to make such comments myself.
Sociology has a lot of language for this, but there are few things more powerful than the disapproval of one's peers, virtual or real.
(Of course, 4chan/SA as cohesive social entities are simply fascinating too...)
it's a cultural thing and therefore subject to the whims of the dominant culture at any time
It's also subject to the whims of dang the moderator, he's "the dominant culture". He's deservedly called me out a couple of times. Consequently I've tried to change my occasionally boorish behavior. Sometimes it only takes one person to create a culture.
One person only scales so far / for so long though. If BusinessWeek wrote an article claiming that "Hacker News was the place to hang out if you want venture capital!" then I wouldn't have optimistic illusions of cultural persistence.
There's plenty of topics here that are worthy of outrage and posted and discussed exactly for that reason. Fortunately people are civilized enough not to stoop to insults and threats. Those are the real problems. Twitter has no method to address insults and threats, and that's its ruin.
One of the reasons I like HN is that outrage is discouraged.