[Jtsummers]

I suspect it helps that people can't downvote until they have 501 karma. That means they must've participated for at least a decent amount of time in the community before getting any real power (upvoting is useful, but downvoting is regulation). By the time someone gets to that level, they know what's encouraged and discouraged by both the moderators and the community.

Another thought: Unlike Reddit or most standard forums, we don't have subfora. We have one forum, one set of front page news, one set of comments per article (with good efforts to minimize duplication of posts, even topics). I think this helps to force people to play nice with each other. I can't go into hn/politics and troll, then go back to hn/programming and be civil. The whole community sees (or can see) my posts. Uncivil behavior in one thread will color the way people view me elsewhere. Forcing me to more seriously consider my behavior (I've deleted many posts, the two-minute delay I have on mine has saved me a few times from posting-in-anger).

[totony]

The minimum karma to downvote also accentuate the echo chamber

EDIT: I guess echo chambers actually help popularity