Ask HN: What have you learned building a threaded comment system?

I'm building a threaded comment system for a web app I am working on and would like to tap into the community wisdom on best practices for making a comment system that works.

Technical implementation tips are of as much interest as the (social) mechanics that ensure high quality spam-free discussion.

The main examples I'm drawing from are HN and reddit, but you can only learn so much by reading their source code.

It's been a while since I've seen a thread on this, so I'll ask:

What have you learned building a threaded comment system?