When they were evaluating Beverly Sills for the Metropolitan Opera, they applied the traditional rules, and generally said no to her.
There are two things: voice and style. Beverly Sills had a fairly light, high soprano voice with a fairly wide vibratto that makes it a bit hard to tell when she's trilling. It's a voice probably most suitable for Daughter of the Regiment, a performance of hers you can buy on dvd. It's a pretty but fragile voice. She is in the same Fach (classification) as Roberta Peters, and no doubt the Metropolitan thought they had the better singer.
That's because they ignored the other thing: her style. Beverly Sills rose above her category. If she had stuck to Roberta Peters' repertoire she probably would have sung much longer. She had the voice of a soubrette and the heart of a heroine. She sang Joan Sutherland's repertoire (I Puritani) and Mirela Freni's repertoire (Manon) because she simply could not resist, and since she was based at City Opera, she could give in to this impulse. It is impossible to imagine Roberta Peters in these roles. She reaches beyond her grasp, succeeding in her heart if not always in her voice. You can hear in her work the heart of a lion.
This is a review of Beverly Sills - the Great Recordings.