At LinuxCon North America 2011 Clay Shirky mentioned Stack Exchange and Stack Overflow. In Clay Shirky Says Good Collaboration is Structured Fighting, Joe Brockmeier reported from the presentation:
As an example of putting barriers to contribution, Shirky pointed to Stack Overflow [Stack Exchange, Area 51]. In order to test whether a new site would work well, they don't just let people try anything to see what sticks. People have to propose a site, demonstrate the type of questions, and then recruit users. Sites that don't make the criteria don't launch, but the ones that do launch tend to be more successful.
New users can't participate easily either. Shirky talked about the karma system for Stack Overflow, which requires users to show up and listen at the beginning – and evolve to an admin-level status that can manage the site.
And somewhat more verbose by Ruth Suehle in Clay Shirky on structured fighting, technology, and all the squishy, human stuff :
Stack Overflow has implemented a karma system, like countless other participatory websites. And, also like many others (including opensource.com), they've added a layer of badges to the karma system. But unlike most others, the badges aren't just "achievement unlocked." They're using karma to enforce restricted participation. When you show up to Stack Overflow as a new user, all you can do is listen. You can't post a question or answer. You just have to be quiet, read, and see how the community works. Newcomers to many communities receive this advice-- take some time to observe before getting involved--but few heed it. Stack Overflow enforces it.
On the other end of the spectrum, at 10,000 points, you essentially become a moderator. You're given the ability to shut down things you consider inappropriate without being a part of the site's management. Once you've proven you're trustworthy, you're rewarded with the ability to act as if the site is your own.
Is a video of Clay Shirky's presentation available somewhere?