Here are the criteria I use when I metamoderate:
When something's moderated anything positive (Insightful, Interesting, Informative), I give the benefit of the doubt. Unless it's blatantly wrong, I'll almost always call it "fair". I recognize that what is insightful to me may not be insightful to others, and vice versa.
When something's moderated negatively, however, I come at it with a built in skepticism, and am inclined to metamoderate as "unfair" unless the person has given me a good reason to think it's fair.
In other words, if something is marked as "troll" and is not someone posting goatse.cx links, etc., is a legitimate opinion, and is not something that most would agree with, I'll still moderate it unfair. I think you have to have a *good reason* to moderate something negatively.
If everyone else would also follow these rules and metamoderate often, maybe the worst of the moderation abuses would go away.