The source of the negative sentiment may boil down to the cold calling. It seems to be one of those things that just hits a nerve, and I wouldn't be surprised if you could trace the anti-Yelp movement to this business practice. The other thing people complain about a lot is that their reviews aren't counted (presumably because of the anti-spam filter). If Yelp fixes those two major problems, it will be in much better shape.

My bad, my bad, what I should have said was something that allows businesses to opt-out of the service.