I wonder what the direction of causation is. Perhaps the issue is not the cool, thoughtful, and kind are pushed out of the forums, but they are successful in engaging people on a personal level and are less motivated to go public. The obnoxious, however, however tend to get avoided on the personal level and find it easier to get an audience in a public forum.
Generalized, this explains why a lot of the really cool, thoughtful, and kind people I know seem to fly under the radar.
And it suggests that moderating a discussion forum means constantly pushing uphill.