On the reverse side, you must also identify your best posters. It's easy to forget while dealing with the obvious problems with crap, but you must find the content that is highest-rated, that draws the most traffic, that most contributes to your business goals. Praise them in public. Send them swag. Make sure your customer service system highlights them, so you're responsive when they have problems. The best users are around 100x more valuable than average. Identify and cultivate them.
You can push the problem around with technical features, but I believe this is a meat-space issue and needs to be treated as such.
Hacker News (so far) is one of the best run sites I've seen in this regard. My belief is that the community is focused on a small enough segment of content that it doesn't bring in contrarian intellect points (i.e. OMG LOLCAT, political bickering black holes). Explicitly defining a charter for the site -- giving users a target to moderate to -- is probably the single best thing I can think of.
You could also let users tip content, and get the data from tipjoy as another ranking tool. Email me ivan@tipjoy.com for more info.