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.
I'm Done
?discussion
tag.not sure
them. All of the questions needed editing. If you're looking for answers to "what should I do here" feel free to look at the revision history for those questions.