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.
My experience is that you can speak about nearly everything here if you keep it civil, relevant, and substantial. Being nice and polite helps as well. There are exceptions of which I sadly have made my share of experiences (and my own mistakes!), but I still consider HN to be a remarkable place.
I wonder to what extent you've tried arguing positions that are unpopular with the San Francisco set.
Yes, HN has good comments on average. But my experience of moderation has been that it often seems wildly random and can be very politically biased. There was a story some time ago about the Guardian and when I first looked at the comments section, it was full of comments describing disappointment or mocking what sort of paper the Guardian had become. A few hours later it was a morass of greyed out, flagged and dead comments ... apparently some voters can't take criticism of that paper no matter how civil. One of my own comments on that thread got downvoted to the min and it consisted only of links to stories the Guardian had actually published, so was pure fact!
Perhaps it got a bit better lately, it's hard to tell, but HN's community is not a representative slice of opinion around the world and it's kept that way deliberately. This is especially true of anything that crosses into criticism of modern identity politics or feminism, even if the contents of the comment criticise ideas and not people.
Indeed doing a Damore on HN is a fast way to get lots of min voted or dead comments and a ticking off from the moderators for "engaging in ideological battle", although of course people posting the opposing ideas are never "engaging in ideological battle". The politeness, civility or level of research involved has no impact on this: it's purely about position.
although of course people posting the opposing ideas are never "engaging in ideological battle"
To me this bias is quite evident, although the mods certainly don't see it that way.
E.g. I once tried to submit a story from Breitbart. It had to do with soon-to-be-laid-off US workers being replaced by H-1B. But of course the hate here for Breitbart is strong, so my non-political story couldn't even be submitted. Breitbart is shadow-banned in its entirety. Submissions don't appear in "new" except to the submitter (and maybe if showdead is on?).
Fine and good. Their web site, their rules. But at the same time, the mods were cool with allowing submissions linking to rt.com. The hate for conservatives / right-wing is so strong here that mods were more accepting of stories coming from Putin's government-funded international propaganda network.
I haven't seen any submissions from RT lately, so I think it has now also become shadow banned. C'est la vie.
Community moderation has also the risk of creating an echo chamber.