The first thing that comes to mind is how rankings would be determined. It sounds like all miners would have to agree that a given rank is valid. If there was a deterministic ranking method, a sort of DoS attack could be possible whereby one creates pages that satisfy the ranking criteria fully but have no usable content.

edit:
If you'd like to see some empirical evidence of this attack, look up search engine optimization. It turns out the economically rational action is to game the ranking algorithm.