It would need to be at the end of a dungeon, after you've beaten the boss. A screen pops up for each player asking you to vote up your fellow players if you wish. Others can't see your votes. That way you'd need to have good behavior and play throughout. And making votes hidden avoids the issue of anger at the end of the DF group if you find you did not get rated up.

At the end of the week you could go to a specific location, maybe your GC, and be informed how many MVP points you gained that week. And make it so that you need an incredible amount of points to get good items, if they do give items as rewards. That would encourage people to redo dungeons to get items, plus encourage good behavior.

I'm willing to give it a chance. It has potential if implemented properly.