Lots of sites think they need to put up walls to maintain a sense of community.

History all across the Internet (and before the Internet) has shown this to be a fallacy.

Tragedies of the common are avoided by proper structure and accountability tools, not by erecting walls to keep people out.

You are over-generalizing, and more to the point - you are clearly neither a Forrst or Dribbble user. The difference in the quality of the content is very noticeable. To put it into a more sensible coordinate system, Dribble is a moderated mailing list, and Forrst is an unmoderated one. Or, better analogy perhaps would be hackers vs. script kiddies - do you know of many hangouts of former that tolerate (leave alone embrace) latter? Me neither.