Comparing Hacker News with mainstream and general discussion subreddits is misleading, as is comparing it to Reddit as a whole. Obviously Reddit is going to come up worse in every conceivable metric (although, also, better) simply because of scale.

A more accurate comparison would be between HN and niche subreddit of roughly equal size and traffic. Not that I am or am willing to put in the work to make such a comparison, but I suspect the quality differential then would be smaller than a lot of people assume.