I'm a firm believer in rating books the way you want. Posting all shiny happy reviews isn't always a good thing. I know people that do that, and I'm fine with that, but if everyone did that, well, that almost feels like lying, or at least withholding evidence.
What I don't like are bad reviews where people bad mouth an author...or just come across as childish. I read one for a cozy mystery where the reviewer was going on about how she never would have read the book if she knew the author had written for Harlequin.
I wonder how many people know how many great authors started out writing for them? Nora Roberts, Jayne Ann Krentz and Debbie Macomber.
But my point is this, if you are writing a review that is less than 3 stars you should give reasons for it. Support why you didn't like it. Don't sound like a petulant child.
Bad reviews suck for the author and other readers when they aren't written well. If you are posting something on the internet, on a blog or website...other people are going to read it, and possibly make decisions about a book as a result.
I'm not saying not to write a bad review, but dang it, offer some evidence as to why you didn't like it that relates to the book.