The Review Queue is not always the first place I visit when I hit a Stack Exchange site. I usually dive into the questions to see if there's something interesting that I can answer or something else obvious to be acted upon.
When I do get into Review and start looking through the First Posts queue, I'm presented with a question on which I've already voted, commented on, and edited. The I'm Done button is, unfortunately not lit.
I don't want to make further (read: unnecessary) edits, so my only choice is to click Not Sure.
It would be better if the I'm Done was live if I've already taken some action on the post.
Alternatively, just don't show a New Post in the queue if I've already taken some action on it.
(This was alluded to in the comments on this answer, but I thought I'd make it a formal feature request.)