From Dr. Sanity:
What made America 'exceptional' is the moral and political vision of its Founders, who clearly understood that a country established on the principles of "life, liberty, and the pursuit of happiness' was, not only an historical anomoly, but also represented the embodiment of the highest values to which mankind could aspire.
What continues to make America exceptional is the extent to which we still live up to those same values bequeathed to us by a group of exceptional and extraordinary men.