There's little barrier to entry, but for the most part the community does good internal policing. Stupid emotional comments are pushed down, and the worst ones are flagged. (I've had a few moments of weakness here, so I'm not trying to be sanctimonious here)

I see a lot of people who conflate "my opinion, which is the correct opinion just won't fly here, so how can you say the community does a good job of self-policing?" I really don't agree with this. Any community is going to hold some opinions you disagree with, and will hold some bad or even wrong opinions. What I generally (but not always) see is HN upvoting comments that are thoughtful and intelligent, not necessarily ones that I think might be correct.