I believe it's a difficult art to write a well balanced review, I would say that as I 've tried to do it myself!
The think the trick is to give the reader as much backgorund as you can without giving too much of the plot away, you should want the reader to go and see the film!
The bulk of the review should be given to how the reviewer feels the film has been acted, scripted, directed etc. If the story has been translated from a book or novel, has the film been able to convey that same message and treated the characters in the same way that the author did.
I have found that after reading some reviews by certain people, you will start to develop a feeling for whether or not you think along the same lines and there fore will agree or disagree with their interpretation of what they are writing about.