Yeah, I agree with everyone - I totally see the point of having a rating system, but it is annoying when people lower the stars without any kind of explanation or, worse, lower them after leaving a completely glowing review.
On fanfics.org, you rate a story based on five areas: writing style, theme, enjoyment, faithfulness to genre, and evaluation. I'm not sure those are the five areas I would choose for rating cause some of them are sort of vague LOL, but I like the principle of it.