the reason comments are good here is because the moderators are serious as a heart attack. you don't get to make many jerk-like comments before your account gets banned.
the other reason is that this community started out as extremely self-selected - people who got through a pg essay or applied to yc, and appreciated the quality that was here already w/o being turned off by the sparse visuals. As more people have joined, that quality is getting diluted, but there's a long way down before it reaches current reddit levels.