“The Metropolitan Opera has to be discouraged by the mostly negative critical reactions to The First Emperor, Tan Dun’s ambitious opera, which had its world premiere at the house on Dec. 21.” But the very fact that the Met is embracing newly commissioned work at all, and the further fact that the entire run of Tan’s opera is sold out, shows that old notions of opera lovers’ conservative nature are overblown, says Anthony Tommasini.