The comments on Hacker News are high quality because there are only certain people with downvoting privileges. And trust me: they downvote ruthlessly. Which is imo a good thing. Just take a peek at Reddit: subreddits with a high moderator count (which actively moderate) are high quality. /r/AskHistorians and /r/AskScience spring to mind. They do not allow any jokes in the comments.
I've been lurking for a lot longer than my account has been active (probably a good 5 years now); have never seen what the criteria are for being able to downvote. Could you please enlighten me?
It 'used to be' 500 karma. But I've heard it goes up over time. I've heard people talk about having ~2k karma without the privilege. So I don't really know to be honest. I wish somebody could answer your question more precisly.
This is why I don't visit Reddit much anymore. I appreciate the seriousness of this discussion board even with controversial topics not related to the programming sphere.