There are quite a lot, ranging from code to autokill spam submissions to graphic tweaks to prevent mob downvoting.

In fact, most of the time I've spent hacking news.yc recently I've spent working on this class of features.

I would say HN has maintained a remarkably high signal to noise ratio thus far.

I agree. "Thus far" is what I'm worried about. This is a valuable community and at some point will need defending from the hordes. There are lots of ways to do that, hence my question.

I think it's just karma (and admin policing).

But karma is very linear, and it's a point system so it encourages gaming. What we want is a system where the genuine participant can contribute with value and not be drown out. Hacker News is small enough that i can contribute and by merits of my post alone, maybe hit the front page. That doesn't happen on reddit, digg, or yahoo buzz.

Giles Bowkett had a post a month or so ago pointing out the same thing. PG's response was basically that admin policing should cut off people trying to game the system with irrelevant content:

http://news.ycombinator.com/item?id=196751

Presumably, he's hoping that by limiting the scope of the content, he can limit the number of interested participants and therefore the scale of the site.