They just write what they think and leave it up to the moviegoer to decide what it boils down to. The only automatic assumption you can make about the New York Times critics is that if a review is very short, they probably hated the movie. The New Yorker, considered by many to be the birthplace of movie criticism, is also symbol-free. And length is no indication of like or dislike. But since the magazine doesn't do a full-length review of every movie, some moviemakers might consider it an honor to have their film reviewed in the magazine at all.
Some critics are lucky enough to have great symbols to work with.
I'd take Roger Ebert's thumbs up or thumbs down over the schoolmarmish grading system any day, because it conveys so much. He could also be saying, this is the best movie of the year and you must see it. Either way, the message is go. The thumb has something in common with the dapper little man who serves as the critic's symbol for our rival newspaper across the Bay. They are both emotional gauges, conveying a very human response.