Not every site. The Ars Technica forums have maintained a surprisingly high level of quality discussion over the years. Ars uses human moderators who read every post and enforce the rules.

There's also Metafilter. Or more spcifically, ask Metafilter which maintains one of the highest quality, large communities I've managed to find online.

http://ask.metafilter.com/

The $5 price of admission seems to be beyond most 1337 hax0rz with funy p!cs and LOLZ.