In what way was the government of the United States a "great experiment?"

Americans were extraordinarily lucky to be able to build a new country from scratch. Unlike Europe or Asia, there were no entrenched interests, ancient regime structures or privileged classes that would oppose the revolutionary experiment of erecting a democratic republic that differed from the monarchies of those times. They could choose and set the laws that would govern them. The country was also endowed with vast and generous natural resources. And finally, there were no powerful neighbors to be balanced against.