I used to write reviews for Amazon Vine. We were given a long list of books from which to choose our monthly selections; sometimes I'd get a couple of really enjoyable books, sometimes not. (it can be hard to decide from a very short blurb.) We had to write a review of every book we chose, so sometimes my reviews were not positive. In those cases, I'd focus on what bothered me about the book: the writer's style, the pace of the story, the lack of plot or too much plot, unbelievable characters, bad editing and so on. There were times when I got sarcastic, mostly in cases where the book was SO bad I felt it deserved to be thoroughly panned. Mostly I tried to be constructive and point out WHY a storyline or character didn't work for me, or why the book was boring or otherwise unpleasant to read. My goal was to give readers an honest view of the book. I had no agenda of promoting a writer or a genre. I tried to write the type of review *I* like to read. I don't like the 5-star reviews that mostly are of the "awesomest book ever!!!" type, because that tells me nothing about the book. I want to know if it's a good read - does the story flow, will I be interested in/care about the characters, will I think about it after I finish it? And if multiple people say it's terrible I'd like to see those reviews and find out why they felt that way.
I think it's possible to be critical without being mean. Saying that a book was boring isn't mean, it's honest if that's how you felt. I have read books that I really struggled to finish, but needed to finish in order to be able to provide a review. I have read "thrillers" that were more like appliance manuals. I don't think it does the author any good service to lie and say you liked a book if you didn't. Maybe they can learn something from your review. Maybe next time they'll find a better editor or do a few more re-writes. I've always felt that people who say "i can't give a one-star review, that would be mean!" are totally wrong.
Anyway, my advice is to write what you honestly feel and don't worry about it.