What qualities led to America's exceptional success as a democratic republic?

The qualities that let to America's success are liberty, egalitarianism, individualism, populism among others.

Explanation:

The main quality that let to America's success is it's commitment to liberty in a sense that each and every individual is responsible for his/her actions. This is in line with the principles of equal opportunity and self-determination.

Egalitarianism or equalitarianism is a belief that all people are equal, and the US is a nation of free people that live in a free land that creates opportunity for everyone.

Individualism refers to the qualities of self-reliance and the supreme worth of the individual as well as the quality of self-dependence.

Finaly we have populism which is a philosophy which emphesizes the fight for the rights of the people and the majority against the rights and privilages of the few.