It's the guidelines. By writing guidelines that promote useful discourse you allow everyone involved to learn more about their own biases and shortcomings.
Every person that writes their opinion grows when reading the varied responses (as long as they're open to other people's perspectives).
That's pretty much it. Once the population gets too big and the level of discourse too low, the intelligent people will move to greener pastures with better signal:noise ratios. Slashdot's moderation system didn't save them, and HN's moderation system won't either.
I've yet to see a comment system that promotes signal:noise ratio over groupthink.
Doesn't every groupthink community describe themselves as "Smarter and more insightful commentators"? I'm pretty sure Trump or Clinton voter websites consider their signal-to-noise high and everyone else's low.