What made America great?
First of all, I would like to say that, while not being American myself (unless you count American as those born in the American continent), I'm deeply admiring of American values and history. More than those of my own country.
I wasn't born in America nor live there but I feel every bit as American as those who do.
I consider countries to be largely arbitrarily defined borders on land thereby not being a separate entity as opposed to just its people, but I will nonetheless use the term America to refer to most of the people who inhabit the land but also those who just see themselves as American and share its values like myself.
I wasn't born in America, don't live in America and don't hold US citizenship, but it is the nation and set of values I somewhat identify with so I see myself every bit as American as someone who happens to have been born there.
I think that America as a set of ideals and values is much more relevant than America the nation-state.
The historical period I'm focusing on right here is basically 19th and 20th century America.
Disclaimer: I know I am referring to stereotypes and groups of people but I do so trying to explain things as we see them today. I know that not all Americans are like this and that not all those who are not American are not like this. People are individuals, not the groups they belong to. I also know that there were other groups that probably also helped, but I am listing those which I think were the most important.
America in its infancy was a blend of very hard-working and able people, like Germans, Englishmen and women, Central and Northern Europeans, Jews and also Italians and other Mediterranean Folk.
More recently, East Asians and Indians have shown themselves to be very high performing and hard working people too.
The fact that most Americans aren't indigenous but rather came to America from other places is probably a very very strong natural selector, selecting individuals that, living in other oppressing and backward countries, decided to do something about it rather than wait. This is not dissimilar to entrepreneurs who take risks to produce value for themselves and others.
Immigration is highly selective. Those who immigrate to other countries are, by definition, people who, rather than wait for their situation to improve on its own, take their destinies (and their families') into their own hands and do something about it. Immigrants were entrepreneurs way before it was trendy.
Americans are naturally people who do rather than wait. Perhaps because if you come from an immigrant background like many Americans (early adopters if you will) you are already out of your comfort zone.
This means that most Americans have (or at least had at some point) a deeply ingrained will to achieve through their own efforts is representative of their work ethic.
They rarely expected or felt entitled to having things given to them other than those trade for their work and creative abilities.
Abundance of Land
Having an abundance of land (mostly good for farming and/or mining) could have proved a mixed blessing (picture oil-rich african and south american countries, ripe with corruption and demagogues promising miracle solutions and/or failed ideologies to their people).
Fortunately, in the American case, it seems it hasn't. Although we obviously have no alternative history America to server as a control group, America is obviously as good as, if not more advanced than, most countries on Earth.
An abundance of land has helped America become, in addition to a great industrial power and the home of the world's best universities, one of the world's most productive agricultural economy.
Correlation does not imply causation but logic does. While government is useful in times of war and to provide basic infrastructure, history shows us that outsourcing services to government is extremely inefficient and drains a country of its resources fairly quickly.
Aside from moral implications and the risk of tiranny, large governments stifle growth and reduce incentives for entrepreneurs and businessmen and women to experiment and find new ways of providing goods and service more cheaply, efficiently and with higher quality.
Aside from moral implications and the risk of tiranny, large governments stifle growth and reduce incentive.
As the references I've collected (1 and 2) as well as many other sources show, America was had a relatively small government (as measured by percentage of GDP) from 1900 up until very recently (although it is still a little smaller than European-style social democracies).
Prior to 1900 (18th and 19th centuries) you can imagine it was even smaller than this.
No Natural Enemies (other powerful nations as neighbours)
This is somewhat dubious but I tend to think that America's lack of enemies (particularly early on in its history) was overall a positive influence in America's greatness.
Those who argue otherwise hold that powerful enemies can bolster one's industries, motivation and resolve, as shown by the number of inventions people come up with during wars, which is certainly true.
I, however, think that "external threats" (real or otherwise) are too often used by those in charge to justify opression and impopular measures aimed at their people.
Governments around the world use the threat of an external enemy to convince their people into giving up their rights.
It would be extremely hard for politicians in America (back in the day when communication wasn't very developed; nowadays it's a different picture altogether) to justify any kind of opression and pushing of particular agendas with an appeal to "favour security over freedom" owing to supposed external enemies, due to its unique position as the sole power in North America.