Nothing.
I've seen blog posts and forum posts on the topic. I've seen people complain that if you can't say anything nice, don't say anything at all. I've seen people remark that "karma" will come around in the end. (Um, really? I had no idea that a person *not* liking your book was grounds for them to receive a karmic ass-whooping.) I've seen the tirades on both sides - authors and readers.
Here's what I think: we read books. We either like them or we don't. A good review (and that includes a negative one) will explain why the reader felt the way they did. Even a "one-star" review can be excellent, if it explains why the reader felt the story was flawed.
I personally don't always feel the need to say why I didn't like a book. Sometimes it's just not a right fit for me, and I don't even spend my time sharing a review of it. Instead, I donate the book to the library or thrift store and move on.
Likewise, I don't complain if I receive a negative review. A person has a right to their opinion. Many things in life are subjective, and that includes books.
You didn't enjoy something I wrote? That's totally cool, and thank you for taking the time to even read it in the first place. I don't blame you for not finishing it, so you could move on to something else. I've been there, done that. :)
When I write, I do it to entertain people. If a reader is entertained and gets lost in the story, I'm thrilled! That is precisely what I hoped to accomplish. If a reader cannot get past some thing or another about the story, then I'm all for moving on to something else (and, yes, they have a right to say "I just couldn't get into it).
I think it would be nice of writers stopped focusing so much on reviews, and paid more attention to writing a better story with each new project.
Copyright (c) 2011 Wendy L. Callahan