Below is the link to an article on Amazon reviews that I think anyone should read before they post a review. There's a lot of useful information here, but a couple of the more salient points:

"Amazon reviews, which were only mildly significant three years ago, now have a make-or-break impact on an author's sales... Rating has a direct influence on the success of a book... and the future failure or sucess of authors...

The weight of a negative rating might do more damage than intended, so better think twice before giving a low rating. In case of doubt, silence is best...

Three stars and less are read by Amazon algorithms as 'not recommended' and will hurt the author. If you liked the book but had reservations on some points, better give it a 4 stars review and then explain in your review what the weak sides of the book were....

A bad review is forever. This is not the place to vent anger mindlessly."

http://www.epublishabook.com/2012/07/20/amazon-book-reviews-10-cardinal-rules-of-using-reviewing-power/#ixzz24efZqU7K