"HN works fairly well because of the hard work of the moderators."
Moderators and size limits, the latter keeping the amount of work small enough that a couple of moderators can handle it, and aren't getting subjected to the sort of stuff Facebook moderators deal with. Obviously, not having images or video also helps that. (Though I recall some times when Slashdot trolls were taking some good swings at Can't Unsee even with those limits.)
HN is on the upper end of what a community structured in the way it is can handle, I think, and it has taken some tweaks such as hiding karma counts on comments. I'm not deeply in love with reddit-style unlimited upvote/downvote systems... in their defense, they do seem to scale to a larger system than a lot of alternatives, but it comes at a price. I do fully agree it tends to create groupthink in a community, as a structural effect, though I think that's both a positive and a negative, rather than a pure negative as some people suggest. Some aspects of "groupthink" become "community cohesion" when looked at from another point of view.
Never thought about it that way, but maybe that's why a reddit-style karma system does tend to hold relatively large communities together.
But even as one of the more scalable known systems, it still breaks down long before you hit Facebook scales, or "default Reddit subreddit" scales.