I consider myself to be a rather thoughtful person. I value what I have to say quite high, or else I wouldn't be saying it. Hence, whenever someone doesn't like what I have to say, I'm very fond of finding out why they didn't like it, and I assume that to some degree everyone has the same interest.
For this reason, I'd suggest to add a feedback feauture to the review voting system: When approving or disapproving of a certain review, the voting user may type in a couple of lines to tell the reviewer why they didn't find the review helpful. These comments are shown (anonymously?) to the user who submitted the review and can be deleted (the comments, not the actual votes) by that user in order to get rid easily of unwanted flames or non-helpful remarks.
For example:
User sees a review that's actually not that bad but way too long to make a point, so he clicks on "No". The page reloads and the vote is recorded, and instead of the No-link he sees a small form where he types in a short feedback message. After submitting that form, the user can neither change his vote on the review nor submit another feedback.
