“[I]t was no surprise that he received a prolonged ovation when he first appeared in the pit at the Metropolitan Opera on Friday night to conduct Wagner’s Tristan und Isolde in his long-overdue debut with the company. The ovation was greater and richly deserved five hours later, when Mr. Barenboim took his solo bow onstage. Though his performance of this challenging masterpiece was rhapsodic and impassioned, it was also keenly sensitive to the score’s harmonic shifts and architectonic structure. The Met orchestra played splendidly for him, and somehow he altered its sheen and color, making it sound duskier and warmer.”