The moment a social news site begins to degrade in my opinion is when comments are posted that add little or nothing to advancing or starting conversation (for example "Modded up for _______", "Really Nice _____" or my favorite "When I read the title of this post I thought it said ___. I have had too much ________ today")
There was a period where it felt as though the same articles would sit on the front page all day. The increase in the gravity for old posts and the increase in total number of posts has caused me (and I would assume others) to become more active in the site. I think the focus in moderation should be on the comments to set an example for what is expected.
Maybe an informal etiquette reminder displayed with a "confirmation" for potentially bad comments. The downvote never hurts either.