The only one I can think of that's stayed relatively high-quality for a long time is Hacker News, and they actively discourage large influxes--for example, by flooding the front page with posts on Erlang internals when mentioned in the mass media.

Paul Graham also does very active experimentation with HN's reputation system, which I like: There are karma threshholds for voting down comments, higher ones for voting down posts; you cannot vote down a direct reply to your own comment; you cannot vote down a comment more than a few days old (this one wouldn't work as well here). The most radical change he's made is that only you can see the exact karma for your own comments (although comments below zero are progressively lighter shades of grey).