In any publicly-accessible forum, spam and trolling are a huge issue, consuming most administrator-hours and requiring a larger staff than would otherwise be required to run a large site. In comparison, sites that charge for admission, and sites that cannot be joined without an invitation have close to no administration overhead beyond maintaining the code, servers, and resetting forgotten passwords.

Some public sites (Stack Overflow, Slashdot, and Reddit come immediately to mind) have implemented a form of reputation. This allows other users to know at-a-glance if the person they're dealing with is stupid, full of shit, or fuckin' nuts. The goal is usually to keep track of troublemakers, but this system can also be used to find the best users on your site. These are people who probably want to help you, given the opportunity.

Here's an idea: pair the "report" function of a site with another reputation, one that is a ratio of number of posts this user has reported to how many of them were valid¹. For users that frequently spot problems with a high level of accuracy, it's safe to take action when a few of these high-reputation users flag the same issue independently instead of making the issue wait in a queue until a moderator can get around to it.

This sort of merit-based promotion of volunteer moderators could be automated and invisible to non-administrators, the high-reputation users not knowing their own augmented abilities, just that the site is a better place for their effort.