American "exceptionalism"

An American left-wing commentator, Glenn Greenwald, writes in the British Guardian newspaper on an important topic, "American exceptionalism." This term gets used various ways; it refers to America being different from other countries (an "exception") and was used in the past in attempts to explain why Marxist, Communist and explicitly socialistic political parties had not formed in the US the way they did in Europe and in Latin America. (There have been such parties, but they have been small "third" and "fourth" parties and have rarely been serious competitors with the Democrats and Republicans.) Greenwald wonders whether the idea has gotten out of control, citing American commentators -- especially conservatives -- who talk about America as the greatest country in the world, or even the greatest in human history. I'd be particularly interested in the comments that Europeans like yourselves might have on this question.