New York City Ballet is marking the 100th birthday of George Balanchine with a season of tributes. Tobi Tobias writes that something seems lost in the celebrations. “I believe that a good part of the problem is that no one is as deeply and unremittingly concerned as Balanchine once was with the nature of individual dancers’ specific personas and gifts. He was acutely conscious of such matters when he created his ballets and when he recast them over the years as well. He developed his dancers not merely in the classroom but through the roles he gave them. Nowadays, casting often seems thoughtless.”