On the one hand, there's honesty. If a reader comes by my blog and picks up a book recommendation from me, I want them to trust me. Heaven forbid they read the book that should have been a two star and wonder what I was thinking when I wrote the review. On the bright side, as a free-lance editor, I haven't picked up a manuscript or book yet that I couldn't find a few good things about. It's easy to play up that. But playing up the good things does not for a good book make.
On the other hand, there's "if you can't say something nice" and plain old karma. I don't like posting bad reviews, period. While a reader would appreciate my honesty, I'm sure, the author might not. Oh, sure, we're all working on getting that tough skin, but it doesn't feel good to be on that side of the criticism. So far my solution has been to only post reviews of books that I give four or five stars. But does that help? Certainly not if everyone did it! I know one person on Twitter who removed all his reviews from Goodreads because he didn't want to post negative ones and didn't think it helped to only post positive ones.
So what's your take? What reviews do you post?