Yelp's business model doesn't sit well with me. Even if the allegations of good reviews disappearing for businesses who don't pay up aren't true, the idea that there is a service out there that you don't want but you're almost forced into paying for since it's affecting your business directly feels to me like paying the mafia to 'protect' your business.
If Yelp really does want to help both businesses and consumers, give businesses the ability to respond to reviews for free and don't artificially alter results, be like Google and let your algorithm work. Make money selling ads.
It reminds me of the whole GetSatisfaction drama with 37 Signals from a few months ago (http://news.ycombinator.com/item?id=540540). This type of business model where you provide a business a "service" they didn't ask for then try to charge them for it will never go over well.
[Edit] FWIW this page, found below in relme's comment (http://www.yelp.com/myths) directly answers every one of my concerns with Yelp. If it's true then great, sounds like they're doing things well. I find it hard to write off all the complaints as fictional though.