Joan Acocella has weighted in on this in "Bring in the Ballerinas: ABT's Guest Policy" -- in the latest issue (June 25) of The New Yorker.
The article is not available on-line, though an abstract IS available, here:
My eye was captured by the last few paragraphs, which includes a review of Natalia Osipova's Firebird ...
She darted, she pecked -- she was the avian sister of Kitri. Of course, a Firebird has to be like a bird, but to move us she also has to be like a human being.
That didn't happen until the second night, when the role passed to Misty Copeland, an A.B.T. soloist. Copeland's great virtues, apart from a strong technique, are deep-digging movement -- the seems to scoop the air -- good humor, and sexiness. So she was able to offset the Firebird's birdy qualities with something more lush. It was really not until she took over that I saw the erotic quality of Prince Ivan's capture of the bird. As the Firebird struggles in his arms, she repeated arches backwards, in what looks like abandon. And when he gives up and stops chasing her, she changes her mind and goes after him .... This gives a powerful undercurrent to the drama ... And it was just like Misty Copeland to bring it off.
Which is related to the guest-star business. Copeland was second cast; Osipova was first-cast. ... That's the way it goes when you have guest stars. Because of their presence at A.B.T. the principal ranks are full. But room should be found for Copeland, who has been a soloist for five years now. The company should have started pushing her hard long ago, partly just in order to help achieve the ethnic balance that classical companies so glaringly lack. (She is the only highly placed African-American woman in ballet in the city.) Now they should promote her for artistic reasons as well as political reasons. She deserves it.
I have never seen Copeland dance, but I relate to the idea of "deep-digging movement ... good humor, and sexiness," and human lushness. (That is how I remember Balanchine's Firebird being danced in the old days.)