I usually give positive moderations lots of slack - I may think a joke or an article is lame, but if somebody wants to moderate it Insightful or Funny, fine - I'll mark those moderations as Unfair/Unfunny if they're promoting obvious trolls or whatever, but that's not common.
Negative moderations are usually obvious also, but the one that I don't give much slack is "Redundant". If an article really was duplicating existing content at the time it was written, or is just adding a content-free me-too, then it's redundant, but if it's a +1 article written two minutes after the main slashdot article, and somebody posts something similar but much more insightful an hour later that makes it up to +5, the first one is still Not Redundant. Maybe it's Overrated, maybe it's Flamebait, and I'd let those moderations through, but I'll call a "Redundant" as "Unfair" if it wasn't redundant enough.
"Flamebait" gets a lot more slack - sometimes there are articles that I strongly agree with (even if I've written them myself
The one meta-moderation I have trouble with is when somebody rates something as "Funny" that looks like it was intended to be serious, not funny (and wasn't accidentally funny either.) Does marking the moderation Unfair undo the moderation, decreasing the posting's status? Or does it just ding the moderator's karma, which is fine...?