Bernstein spotlights some of the fault lines running through the American musical establishment, and the centennial makes it clear that, in spite of his example, they haven’t changed all that much. Ironically, the classical music world will be feting Bernstein in part for his role in merging the American vernacular with high-art music, in works such as “Candide” and “West Side Story.” But throughout his life, critics castigated him for not being serious enough.