It may be an obvious point, but I've known it ignored often enough....if you don't like a book, do explain why, because that helps readers decide if the book is for them. I have known reviewers comprehensively slag off a book simply because they didn't like the story, which another person might well enjoy - tastes differ, after all. So try to separate more objective criticism of (for example) the writing quality or the characterisation from your personal preferences. In other words, be fair to the author!