I think the biggest problem with Yelp is fake reviews intended to harm a business either from competition or disgruntled ex-employees. They allow 1-star reviews

I think the biggest problem with Yelp is fake reviews intended to harm a business either from competition or disgruntled ex-employees. They allow 1-star reviews from throwaway accounts to stay on the front page and filter 5 star reviews from active users unless you pay for their service which includes 'account management support' to dispute fake reviews.