He was the first American-born conductor of a major symphony orchestra to become as famous as he did. There had been some American-born conductors before him, including Arthur Fiedler, who conducted the Boston Pops from 1930 to the late 1970's, and Alfred Wallenstein, who became conductor of the Los Angeles Philharmonic in 1943.
Fiedler did become very well-known, but the Boston Pops played, and still play, mostly light classics, not pieces like Beethoven's Ninth Symphony (as the N.Y. Philharmonic did), and none of the other American born conductors had even a fraction of the impact that Bernstein did, although the Boston Pops' recordings have always rivaled Bernstein's in popularity. Bernstein's many talents - conducting, composing, writing, teaching, and piano-playing - aroused the admiration of the public, but resentment from a few critics.
It was not until Bernstein was into his later years that some critics who had previously dismissed him began to show a grudging respect for him. Today, he is universally acknowledged as perhaps the greatest conductor that the United States has ever produced.