Well, that gets to the heart of the matter, doesn't it? There's no such thing as a single dimension on which we can rank players -- it has to be a ranking of something. That something could be the guy's ceiling, it could be his "mean project", it could be his trade value.
But so long as we don't ever say what that something is, ceiling will play a very significant role in these sorts of polls thanks to our optimism as fans.