There are pros and cons to each method, Jon. The con to what you are suggesting is anonymous flaming of stories. I think that would be far more common.
Furthermore, and I am sure this sounds somewhat arrogant, not knowing who offered a positive or negative comment would not allow you to take the proper consideration. For instance, there are certain reviewers who offer incredibly good reviews for stories that are complete crap. The converse is also true. I find it far more helpful to get a scathing review from someone of intelligence than to get a wonderful review by an idiot.
In other words, I completely disagree with your suggestion to allow anonymous reviews.
Finally, if you want reviews - ask for them. Create a thread asking for a review. You will get reviewed. Also, check the reviews of other stories. Find a few critics who seem to make fair, clear and explanatory reviews and who seem to really care about writing. E-Mail that reviewer and ask them for a critique. I think you will get what you are looking for - well, sort of, anyway.
Well, that's my dollar and a half - long winded bastard that I am!