It is expected that the story will hang together well as an artistic whole. This standard of film would have something special about it: perhaps something original in the treatment of the subject, or something out of the ordinary about the content, or be technically creative. It may, however, have minor flaws which detrimentally affect the audience appeal.

Clearly each rating will inevitably cover a range of attainment levels, and will be subjective to some degree; film is a work of art not science.
Please also be aware that the general standard of films drifts over time.