Most critics from the Online Film Critics Society (OFCS) enter their own quotes and ratings. For critics who don't enter in their own quotes and ratings, it's basically up to the judgment of the editors. They take into account word choice, rating (if any), tone, and who the critic is in their determination of whether a review is positive or not.
If an editor is not certain about a review, it is sent to another editor for a second opinion. "Wishy-washy" reviews or reviews where it's difficult to determine if the critic recommends the film are usually given a Rotten rating. We figure if the critic is not confident enough to give the movie even an implied recommendation, then we shouldn't either.