Hacker News is a "forum" whose main purpose is getting people to talk about/think about/post links to information related to startups. It has a very explicit purpose and to this end I would not expect it to be "infested" to the extent of a lot of other forums.
For instance, would we expect to see a lot of trolls on a forum devoted to advanced topics in theoretical math? "Orbifold cohomology is so much better than Hopfschild cohomology!" No. No, we wouldn't. A serious, well-defined, topic begets serious discussion.
This is the real problem with reddit, digg, etc. - there's no clear GOAL. Like, there's no -reason- that people shouldn't be assholes. What are they getting in the way of?
It's like if you opened the doors to your house and just held a "general forum." At first, discussion would be great (it's just your friends). Later, discussion would get worse (it's your friends' friends). Eventually it would degrade into the biggest losers who have nowhere to go sitting around and making fun of all the fun they're not having out in the real world (outside the house).
I agree with pg's sentiments that the architecture/rules of hacker news make it a much more intelligent, even (gasp) friendly forum, but I also think it's the community. They keep coming back because they have active interest. They have active interest because hacker news has established as its goal the provision of things of interest.
As always, I might just be woefully naive about human nature; but things seem so simple!