Scale can reveal problems. Overall, discussion on HN seems to be higher in quality than when I made my first account in 2012, but there are a lot more people. The number of people who upvote good comments seems to have gone down though. It makes sense: there are a lot more good comments (in quantity and probably in proportion), so it's easy to get tired of reaching for the upvote button on all of them. Meanwhile, people who make snap downvotes for ideological reasons still reliably downvote. A nerve-touching comment can easily hit the -4 cap without an equal number of upvotes to balance it out.

If I'm right about my theory of the reduced propensity to upvote with a higher quantity of good comments, there could be a tipping point where quality of discussion does go down as good but controversial comments sink to the bottom. HN isn't there yet, but it's something to watch out for.