[ebbv]

This just isn't true. Every site I've ever seen get above a certain threshold of popularity has its comments site turn to utter garbage, if it wasn't terrible already.

The problem is that once your comments section quality reaches a certain point, nobody wants to bother spending time an effort writing something thoughtful. For two reasons; one it seems like nobody there will appreciate it and two it gets drowned out by the noise that surrounds anything bombastic.

[zigzigzag]

HN, Slashdot and Reddit frequently have thousands of posts on a single story and many of the posts are excellent.

The keys are that all three sites are dedicated to discussion: that's practically all they do (plus story aggregation). So they are optimised for it. A simple example of how they differ from most newspaper comment sections: threaded replies. Posts that span the page width. Karma tracking.