Based on the article and the comments here, the following seem to be consistently agreed upon:

-Yelp "rotates" reviews

-They offer to "sponsor" good reviews for a fee

As far as I know, other sites (I'm thinking TripAdvisor, and for paid sites, AngiesList) do not do this, which makes the whole thing seem a little shady. Doesn't take much to imagine that they would go one step further into shaking down "customers" to rotate out bad reviews.