Starting from the premise that what mods are doing is editing the replies with the objective of making the reader experience better rather then simply 'grading' posts...
+1&-1, unintentionally redundant
Quite often, 2 excellent posts say the same things, and appear moments apart. Clearly the posters put a lot of thought into writing them, and might also have been insightful, etc, and it's sheer bad luck that someone wrote the same sort of thing at the same time. The post is of no use to readers, so 'redundant' should be used, but it's not the poster's fault, so when I'm modding, I hesitate to use a karma-hitting mod point. What I would like is a modding option that gives the poster karma for their good intentions, but demotes the actual post out of the way of readers.
-1, sense of humour failure
All too often I see posts made at +2 that pick logistical flaws in posts that are clearly intended to be funny. They don't add anything to the discussion, and are not really useful to anyone reading the story, but there isn't really a good way to mod them down. I occasionally use "overrated", but it doesn't really fit well or get the message through to the poster.