Here's an idea. On the DailyKos blog, they have a message rating system, but it's much simpler. Basically, there is one button, to uprate a post, making it known that the post in question is a well-regarded post. WHen someone has enough upratings within a past number of hours (or days) they become a Trusted User, and can then also down rate posts that they feel are inappropriate or otherwise detracting from debate. Once a post has enough downratings, it gets hidden and only viewable to moderators and trusted users.
Now, obviously, the formula for deciding when someone becomes a TU and when they lose TU status (not having had enouhg consistently good posts to get upratings) is up to you to decide, as is the penalty for downrated messages.
If you don't like the idea of a Trusted User, seeing as how that may infringe on a moderator's role, you could simply have the uprating as a way to mark good posts.
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