One of the best posts I've read about internet communities discusses this, long but worthwhile:
http://www.ribbonfarm.com/2010/12/07/socratic-fishing-in-lak...
The relevant point is the dynamic balance between "plazas" (highly accessible areas to anyone) and "warrens" (areas accessible only to an accultured in-group). "Evaporative cooling" is the phenomenon of the best contributing members "boiling off" and leaving due to frustration from excessive noise, flame wars, etc. From the post:
"Warrens are like rabbit warrens: low global visibility, highly personalized corners where small groups can aggregate/self-segregate. Plazas are like grand public town squares with lots of global visibility. Warrens are maze-like: it is hard to break out of your current corner, and it is easy to get lost. You couldn’t get lost in a plaza even if you wanted to.
[...]
'Warrenizing' a site undeniably slows evaporative cooling, by allowing groups to self-segregate. On Sulekha, we had a warren of individual topic boards, and a firehose plaza that consolidated the entire discussion stream at the landing page. When flame wars got too hot, we used to simply turn the plaza stream off, so people were forced to go directly to individual corners. Things would immediately get quieter. Groups would get sequestered, and therefore less likely to leave because of unpleasant activity elsewhere that they didn’t like."