Here is an interesting map of the U.S. with the 50 states renamed as foreign countries with a similar GDP. California produces as much as France, Missouri as much as Poland, Illinois as much as Mexico, etc. [ht: acton.org]
America truly is the bread-basket of the world. Not just in terms of agriculture, but in terms of a myriad of goods and services which raise the standard of living for billions of human beings.
What accounts for America's productivity? Historically it was the "protestant work ethic." But today, millions of Americans work only to accumulate "adult toys." And one has to wonder what percent of America's GDP consists of purely recreational items. Not that recreation is bad, but that the emphasis may have shifted from being of service to those in need, to a more selfish and short-term perspective.
In fact, the "work ethic" may have resulted in too much work, detracting from family and faith.
Finally, calculations of GDP may include government production and sales of weapons. Today America is the largest exporter of both pornography and tools of mass murder.
When the Puritans spoke of America as a "City upon a hill" they were quoting the Bible. When that same phrase is used today, it tends to shore up the idolatries of nationalism and materialism.
America is only as great as her God, and if America worships the creature rather than the Creator, then we endanger the prospects of passing on the blessings of liberty to our posterity. A high GDP cannot save us, nor the government that takes credit for it.