The next four paragraphs will seem irrelevant to the question at hand, but they are not, so bear with me for a moment.
I've got a bachelor's degree in math from Caltech. Caltech requires every undergraduate to take a broad range of science courses, so besides math I've got a pretty good science background in general.
After school I became a programmer, and have been a console games programmer, a Unix kernel hacker and porter, an embedded OS architect, wrote reference SCSI firmware and device drivers for a major chip maker, wrote network bridge and router firmware, did assorted consumer software, and have done a lot of back end stuff including a PCI compliant credit card storage and billing system. In other words, I've got a pretty wide programming background.
At some point in there, I took a break and went to law school, taking every intellectual property course and every tax course they offered, and did particularly well in evidence, constitutional law, and first amendment law. About 99% of the way through I decided I'd rather be a programmer who knew a lot about law than a lawyer who knew a lot about programming.
I also read a lot on a wide variety of subjects, and have taken a few MOOCs in areas outside my prior work or school experience. I wouldn't say that I'm particularly educated on such subjects, but I seem to have a knack for remembering the odd interesting fact from such endeavors that will later later turn out to come up in general discussion.
Consider /r/technology. Discussion there usually involves technology, and quite often involves the impact of that technology on law or the impact of the law on that technology. The same is also true of much of the discussion here on HN.
On HN my combination of backgrounds in math/science and law is an asset in such discussions. If I post a long comment full of citations to back my claims, with most of the claims being matters of fact rather than matters of opinion, that will generally either get up voted or will not get votes.
On HN, it seems people like learning things from comments, even if those things might contradict what they thought was true. (This is one of the main reasons I don't mind submissions to paywalled articles that I cannot get to for free--if the article is interesting I'll probably learn something interesting from the HN comments, and people usually provide enough context that not having read the paywalled article is usually not even a hinderance).
On /r/technology, it will get up votes if it backs the popular opinion. If it goes against the popular opinion it will get a lot of down votes, often enough that it is hidden by default.
On many subreddits, people seem to want the comments to be a safe space where they will have their existing beliefs validated.
Because of this when I start to write a comment on reddit I now try to limit myself to comments that will only take a couple of minutes or so to write at most, except on small, specialized subreddits. It's just not worth taking the time to actually do research and fact checking before posting on the larger subreddits because of the risk that the comment will get buried by down votes.
I assume that many of the same reasons that make me put more effort into my HN comments than my reddit comments also apply to other people here. The net result is that HN has a higher fraction of useful comments, and a higher fraction of smart, open minded readers. These reinforce each other, and keep the site viable.
In a way the bigger subreddits, like /r/technology, remind me of high school and HN reminds me of college.
In high school I was almost always the smartest kid in the room (and usually "kid" could be replaced with "person" without changing the accuracy of the statement). At Caltech, I was almost never the smartest person in the room if the room was bigger than a bathroom or an elevator. Caltech was infinitely more fun than high school, because it is a lot more fun to be surrounded by people smarter than you than it is to be the smartest person.
In a subreddit like /r/technology or /r/programming, there usually isn't enough information to rank people more than coarsely than maybe quintiles, but I usually seem to be in the top quintile.
On HN there are plenty of commentators, both regular and occasional, who are clearly smarter than me. I seem to be about average here. That's a lot more fun.