A one-star review can actually encourage me to pick up a book if I can see that they disliked it for the same reasons I'd enjoy it. For example, if they said it was "too dark", I'd definitely be intrigued.
This is another great point. Switch "dark" with something like "romantic", or "had a strong paranormal element" and that might be me. Someone else's complaints might actually end up being advertising, depending on what the reader in question enjoys. Everybody likes different stuff.
Veering off on a video game tangent...
For video games my favorite is "There is too much story." Great! Sign me up! Whereas when a game site says "This game is really, really hard." then I want a demo. I have cerebral palsy and I am legally blind -- games are interactive books to me, so "really really hard" gameplay will not sell.
Getting back to books now, though...
I actually read one star reviews on Amazon for fun. For some reason it amuses me to hear people giving such strong opinions on what they dislike. Sometimes I take note of something, most of the time it goes in one ear and out the other.
I wonder how people actually get started doing a review blog / site. Not sure I'd have the time for it, but it could be interesting. But now I'm rambling about a whole other subject.
Knowing how to read a review in a way that helps you sort it out for yourself is the most important part of the whole process. I'll stand behind that statement.