It seems to me that people are very loathe to leave negative feedback. If one reads people's profiles it would seem that most people on this forum are, if not saints, close to that. This seems unlikely! Particularly when one meets the variety of guests who are out there - some are better, some are worse, and some are certainly not like the impression one gets from reading the feedback on them. Similarly, the stories that some guests tell about their hosts doesn't match up with the feedback about the hosts.
Lack of leaving negative feedback is not specific to this site, it is a common problem for all similar web-recommendation systems. People don't like to be thought of themselves as negative and perceive that someone who leaves negative feedback may be a problem. So many people leave no feedback rather than negative feedback. Or at least put neutral feedback when they have had a poor experience. I am not expecting anything to change as this is all inherent in human nature. However, it certainly limits the utility of this type of feedback system.