I constantly struggle with this as a reviewer and I think I'm not alone in this feeling. Writing a review that is fair and honest is hard, but I find it rewarding when I let people know exactly what I felt about that story. Whether there were plot points that made think or if there were plot holes that made me wonder what happened. It's never fun to write a negative review, just like it's never fun to read a book that maybe just didn't knock your socks off.
The first thing to think of when writing a negative review is to not attack an author. That is just the number one mistake. Perhaps this book just was not your style. That is not the author's fault. Obviously the style of writing will appeal to other people. Remember your review is just your opinion and not everyone will feel the same way you do.
When writing a review whether positive or negative try to pick out what you liked and what you didn't like. Balance is the key. When reading reviews by famous reviewers, you'll notice they balance the good with the bad. It can be done. Was there something in the text that made you go 'what'? Did you like how certain back stories linked up to main storyline that really made an impression on you?
I've read several books that are targeted to young adult, but I honestly found them to be more middle grade in style and tone of the story. Is that the author's fault? No. Again, do not attack the author.
Editing. Sometimes you just know when a book has been rushed in the editing department. I remember when HARRY POTTER AND THE ORDER OF PHOENIX came out. I read so many different opinions on it and the biggest complaint was that it needed to be edited a bit more cleanly. It was too long, there were storylines in there that could've been removed (again, what I read). The anger of Harry to Dumbledore and his constant YELLING could have been toned down. Again, which would've been done in editing. For one reason or another, they kept the plot lines in and we have another best seller.
None of us like to write negative reviews, and it really isn't up to us to tell someone not to read that book. If I write a negative review, it is my opinion. You may love that book. That is the whole idea behind writing reviews, so that we can pick the good and the bad and let other people make up their own mind as to what they want to read. Never take my review as the be all and end all to a story. Challenge me. Read the book and if you love tell me why you love it. What did I miss? Would a re-read help? Or perhaps it just wasn't my style and nothing will change my mind.
I'm not a fan of Stephen King. I don't understand his appeal, but obviously, as a household name and an extremely best selling author, I'm in the minority. Yes, there will be people who will think I'm nuts because I don't like his stories. I'm also not a fan of slice and dice movies, either. But there are millions of you out there that will spend money and go see those SAW movies, Freddy Krueger, and FRIDAY the 13th movies. So I already know not to review those:)
These are just a few ideas and points that I've been thinking of. Authors if I didn't like your story. It has nothing to do with you. There was just something that didn't connect with me. Doesn't mean that someone else won't connect with it.