Some ideas....

I think I should point out that on the site where I work we have constant problems of spammers using stolen credit cards to buy accounts in order to send spam. So even paid membership is not necessarily an indicator of goodness.

Personally I favour user-moderation such as Slashdot or StackOverflow. Users with enough karma/reputation can perform certain actions without requiring site admin oversight. This seems to work well on those sites, but I suspect they have more traffic.