I've come across many posts that can only be described as okay (see examples below). They're not good enough to deserve an Upvote and not bad enough to deserve a Downvote, there's no point in editing them except maybe to correct one letter, there's no need for a comment and they don't have spam or bad language to flag.
Because the numbers are decreasing the more you "review" it gives you a some-how rewarding feeling, that you're helping decreasing the problems. But at the same time they're putting a pressure on the user to do something in order to unlock that awesome I'm Done button. So the reviewer might do unnecessary edits or leave unneeded comments.
I suggest having a Looks Good button, and when enough people (3, 4?) think it looks good it can be treated normally.
I don't think Not Sure is the only option we need here, because it isn't the same as Looks Good ("Low Quality Posts" have both). I think that we need something to mark the post as okay. Because I'm not Not Sure, I'm pretty sure it Looks Okay.
I also strongly agree with sixlettervariables, this has discouraged me many times from using those two sections in the Review system.