Have you ever read a review that said the book was "OK?" I haven't. Ever.
This is a bigger problem than most people think, as I see it. If reviewers are either really positive or really negative it makes it hard to figure out whether you will like the book or not. This leads us to a logical question: why read the review in the first place? One sided reviews also make you question the objectivity of the reviewer. Its reasonable to think that most of the books are not at one extreme or the other but somewhere in the middle. I also think this is a problem for authors who need unbiased criticism to get better at their craft.
I think there are a couple of reasons for this. First, I think some of the reviewers are friendly with the authors and don’t want to hurt their friend's feelings with a negative review. Second, some reviewers let their love of YA show in their reviews. They want to promote the genre, not give an objective review.
One reviewer I think does a great job with objectivity and the right tone is The Book Muncher. Her reviews are always thoughtful and suggest improvements to books without preaching.