On the question of American exceptionalism (subscription required), James Bennett puts aside conservatives’ emphasis on abstractions like “freedom, prosperity, and innovativeness” as well as liberals’ emphasis on “America’s unique evil or guilt.” Rather, he looks to culture and history to explain how the United States differs from other countries in a substantive way.
His analysis comes down, essentially, to three factors: family structure, geography, and narrative. On the first, America follows other English-speaking nations in its traditional liberty of family structure. Adults in the Anglosphere have long chosen their own spouses, sent their children out into the world to do the same, and minimized expected structures of inheritance such as primogeniture, the result being as follows:

… The individual in the English-speaking world has always been psychologically more independent and less willing to place himself under the control of others. He expects to be on his own, with a spouse of his own choosing, to make his own way in the world, and if possible to live in a home of his own.

America is then uniquely defined by the effects of the American continent on the variations of English-speaking peoples who arrived on its shores during and after the Age of Exploration:

America’s uniqueness can be explained in two main ways. First is the “frontier thesis” of the historian Frederick Jackson Turner. In the 1890s Turner wrote that early settlers in America underwent a psychological transformation because of the constant lure of open land to the west, which turned deferential, class-conscious Englishmen into egalitarian, assertive, republican Americans. The other view, most recently stated by David Hackett Fischer, is that, in essence, all the ingredients that made Americans what they are today were present when the first colonists left the British Isles. According to Fischer, what the Americans brought to the wilderness was at least as important as what they found there.

Subsequently, the circumstances and methods of our national founding institutionalized these attributes in the legal language of the Declaration of Independence, Constitution, and subsidiary documents. All three factors — culture/family structure, geography, and institutional narrative — have carried the uniquely American character into the present time. One significant consequence is the inadvisability of emulation of picked and chosen attributes of other countries:

The Anglosphere in general is poorly adapted to large-scale, planned, centrally directed state enterprises or invasive measures to promote equality of outcome. Governmental mechanisms have been and will continue to be used on a pragmatic basis, but they are not immune to public-choice problems, as can be seen in the regulatory capture of the home-mortgage industry, or the taxpayer bailout of the auto industry. Our history is filled with short-term successes of government action that eventually succumbed to these public-choice problems and required reform or abolition. The government financing of railroad construction after the Civil War was a scandal-ridden disgrace, for example. When we try to be like the French, Germans, or Japanese, we are particularly liable to poor implementation, because our cultural structures are dissimilar to theirs. Government-run enterprises in those countries are likely to work better than they would here. Even if it were desirable to imitate them, we would not be able to do as good a job.

To put it in analogy, one cannot drive a bulldozer like a motorcycle. Bennett points out, as a contrary example, that the French are more comfortable with meritocracy in government, so the state bureaucracy has developed a practice of identifying talented students and channeling them to itself. One can see an attempt at emulation in President Obama’s plan to forgive the (government-owned) debt of college graduates if they go into “public service.” America’s discomfort with the government’s picking winners, though, requires us to use generic acquisition of a college degree as the evenly applied criterion, while at the same time making college degrees universally accessible. The attempt at institutionalized meritocracy therefore will fail.
Broadly stated, the factors that have fostered the United States’ dynamism do not fit well into a statist public structure. That helps to explain why civic statists are so frequently simultaneously social liberals. It’s possible (if functionally deluded) to be an economic conservative and social liberal; by contrast, socio-cultural conservatism generates habits of mind at odds with economic liberalism. A person acculturated to strive for the good of his own family will resent the attachments of economic dependents to his or her estate without his consent. That same person placed within a bureaucratic milieu will not be an effective socialist because, in effect, his capitalistic individualism will color his judgment.
ADDENDUM:
I’d further suggest — if I had the time to go that far beyond Bennett’s argument, just now — that the American system more closely comports with human nature. That is to say that cultures that are better suited to socialism are merely papering over individualism. Personal interests will ultimately corrupt such systems, leading to economic malaise and civic turmoil.