Liability should not be black-and-white. HN has a process for flagging and burying content, and they employ moderators. If some kind of libel is buried in a greyed-out user comment or a flagged post, then no harm. If the libel sits on the front page in the title of a post for two days, there's a real issue there.
It's interesting that you mention HN, because it's basically not a problem here.
That kind of goes to my question though. It seems that people are suggesting that by offering a process to flag & bury content and by employing moderators, HN becomes a publisher and therefore should be liable for our posts.
I could easily post a libelous but interesting comment here that I'm sure that no moderator or no HN user would recognize as libelous, so it isn't going to get buried.
It's interesting that you mention HN, because it's basically not a problem here.