We ran our forums at Bonanza for about 4 years, and had millions of posts made to them. We spent hundreds (thousands?) of hours of time building tech and training moderators.
In spite of the overwhelming effort we put into making them useful and productive, we ultimately had to concede that the folks who sought to undermine our forums had 100x the time that we did, and displayed a level of resolve (creating duplicate accounts/their own forums/blogs to pick apart our decisions) that could have been a huge asset, if not for being dedicated to subterfuge.
After seeing all this firsthand for years, I am now solidly of the belief that the Internet and productive forums are hopelessly at odds. I hope Discourse can prove me wrong. But all it takes is a small handful of people to undermine the best technology and intentions.
That Paul Graham has been able to keep HN discussions civil for years (even though I've read it causes him great consternation) is an inspiring counterexample. But I can scarcely imagine the number of cycles he and his team probably have to expend to keep the signal-to-noise ratio as high as it is here.