You're not going to like every book you read. It's called "having an opinion," and it's perfectly okay. But a scathing review is as unhelpful as a dishonest one. A good reviewer helps his fellow readers choose books that they will enjoy--and there are a couple ways to can make sure that that is what you're doing.
1. Give Context. Context is the single most important aspect of your review. Taste is relative, so you need to let readers know who the audience for the book is, and what your tastes are. Without context, your well-thought-out review is meaningless. After all, how useful is a four-year-old's opinion on Dante's Inferno?
2. State Opinions Not Facts. You can't compare the book to a Platonic Ideal of Bookness, because there isn't one. So instead of declaring something bad, say the truth: you disliked it. And remember: as much as you think that having a cow crusader called the Masked Moo is stupid and cliché, Johnny over there has never seen anything like it, has papered his walls with the book posters, and goes to school wearing spots and a domino mask.
3. Give your reactions. Even better than saying you dislike something is explaining why. And I don't mean by tearing it down. I mean by describing what made you feel negatively about the character or plot or whatever caused your issue. For example, "When the Masked Moo squashed that innocent little sea slug under his cold, merciless hooves, I just couldn't identify with him anymore. I mean, who does that? It made me feel bad for the Omnipus and his sea slug legions!"
4. Identify what you like as well as what you didn't like. Instead of saying that the Masked Moo was an unlikeable hero, try saying: "I like my heroes brave AND compassionate. When the Masked Moo risks his life to save Mini Moo, I really felt for him, but I just didn't understand why he wanted to squash the poor sea slug." That way, you give other readers a perspective on your opinion.
5. Don't Bash the Author. It's just rude. Remember: they really are trying their hardest to make you happy. Besides, you never know the circumstances under which the book was written (and it's also rude to speculate: "Man, Mr. Mooster must have been hit in the head repeatedly to think this is good!), it might not have been written for you, and judging a whole person by one book that they don't have complete control over is like judging someone based on their outfit on prom night.
Do you have any other good advice for new reviewers?