Reddit and HN make an interesting comparison. Reddit's highs are really high, far higher than HN's, often skirting the level of "so good as to be newsworthy". But Reddit's lows --- and I don't mean trolling and harassment, I mean the lower bounds of normal conversation there --- are much lower. I think the median HN comment is better, by a lot, than the median Reddit comment.

I think you can extend the same judgement out to entire threads --- the best Reddit threads being so much better than HN's that it's hard to even imagine what HN could do differently to compete, and the median HN thread being better than the median Reddit thread.

Another interesting thing is that the comparison would probably fall apart completely if not for the heavily-moderated subreddits that produce the best comments. If the Reddit comment cohorts included only the lightly moderated subreddits, HN would probably win on all metrics other than volume and diversity of topics.