I always thought the biggest failing of Americans was their lack of irony. They are very serious there! Naturally, there are exceptions... the Jewish, Italian, and Irish humor of the East Coast.

This quote is saying that Americans in general have difficulty expressing irony, which is the ability to express ideas and sayings by using words that are the opposite of their literal meaning. The quote suggests, however, that there are exceptions to this, such as the Jews, Italians, and Irish living on the East Coast who typically have a humorous outlook on life.