I've been doing some research lately on online communities related to eBay for a client. Today I saw that eBay is using a member's reputation/feedback as a seller to limit participation in the discussions.
You may post 10 more message(s) in the discussion boards. We encourage you to build up your feedback score to 10 because once you do so, there will be no limitations on the number of daily messages you may post.
If you need to see it in action, look at the bottom of this page.
I thought that was an interesting way to filter the members and keep the riff-raff out of the discussions until they get positive feedback. I like the idea of reputation crossing over from transactions to the community- that a person who is a trusted seller can also be trusted, to some degree, in the discussions.
UPDATE: eBay also has a groups feature that is similar to Yahoo! Groups. To start a new group, a similar, but more strict policy is in place:
To start a group, you need to have a minimum feedback of 50 and have been registered on the site for at least 90 days.