Maria Callas was simply incredible. She understood the flow of the phrase more completely than anyone else. I especially enjoy when a singer can stretch out the tempo and carry the phrase over larger and larger landscapes.
Many people don't like her, usually because of the gradual deterioration of her tone as she got older. My current theory is that she was a pushed up mezzo, and that when she was preparing her spectacular performances, that was the last thing she worried about. The clue is that mezzo roles fit so well into her voice.
What we look for in a modern day performer--at least this is what I think--is that each creates her own unique performance. I look for the heart and soul of the singer who stands before me. Believe me. If there existed in the time since Callas anyone who could work the Callas magic, she would have done so by now.
I tried to communicate my position in my long essay on performing the mad scene from I Puritani. If you prefer to sit home and listen to recordings of La Divina, by all means do. Just don't come around when we're enjoying someone new and trash them because they're not as good as the long dead Callas. We want the new singers to be the best that they are capable of. We want to be thrilled by what they bring to the performance.
I always feel that the less I know about a performance before I hear it the better. I want to be surprised. The easiest way to avoid comparisons with Callas is never to have heard her.