I wonder if a big part of the difference is that HN has only one stream of postings. If there were a HN-politics, HN-tech, etc. each may end up as an echo chamber like reddit.
I participate on HN but only lurk on reddit due to so many upvoted garbage-comments on the latter. I don't much care if reddit posts/comments in futurology are always "woo! yay technology! future!!" and everyone in babyelephantgifs is just celebrating baby elephants all the time, with no dissent.
I do care about endlessly repeated/upvoted noise in comments. Puns instead of topically relevant comments, political sniping completely unrelated to the story, catchphrases from the poster's favorite movies/TV series/games, and basically everything from reddit site:knowyourmeme.com. Yes, I know, smaller subs and content that never makes it to the front page don't suffer in the same way. I do enjoy reading the rust, programming, and AskHistorians subreddits.
