MetaFilter (which I hold as the high-watermark for well-moderated, respectful chat on the internet) makes you (a) pay $5 and (b) wait a week after paying $5 before you can post. Those two things seem to filter out 99% of the twats, and having paid mods who take no shit (and don’t pretend to care about your bullshit free-speech argument if you’re clearly just provoking) takes care of the last 1%.
(like DiS, though, they’ve been hit hard by the downturn in ad revenue and similarly hold regular funding drives amidst constant handwringing about whether the site is still viable…)