As someone who gets quite a lot of shit for his movie ratings, I quite enjoyed Roger Ebert's explanation of how he decides how many stars to give a film and why his ratings are usually higher than those of other critics. I give this bit 4 out of 4 stars:
In the early days of my career I said I rated a movie according to its "generic expectations," whatever that meant. It might translate like this: "The star ratings are relative, not absolute. If a director is clearly trying to make a particular kind of movie, and his audiences are looking for a particular kind of movie, part of my job is judging how close he came to achieving his purpose." Of course that doesn't necessarily mean I'd give four stars to the best possible chainsaw movie. In my mind, four stars and, for that matter, one star, are absolute, not relative. They move outside "generic expectations" and triumph or fail on their own.
His "I like to write as if I'm on an empty sea" line is happily filed away, to be used as liberally as possible.