I've come to realize that the Declaration of Independence, The Constitution and the Bill of Rights are extremely revered in American culture and hailed as a significant advancement in philosophy and law.
In my own (Dutch) background the story is widely different, the texts are placed in a context of Locke, Montesquieu, and and other Enlightenment thinkers and the American applications of their theories, while significant for being an early place they're put into practice, weren't of much philosophical significance.
What's the truth? Probably somewhere in the middle, but how much original thoughts did the Founding Fathers actually have, and how hard was it to come up with the American Constitution?
So, there are two questions here. Let's address the originality of the Founding Fathers (ugh) first. Basically, you have it right here. The intellectual leaders of the American Revolution were adaptive, not innovative. Literate Americans had a voracious appetite for political pamphlets that adapted writers from the Glorious Revolution (Locke) and the English Country Whigs that railed against corruption in Parliament (Bolingbrooke). The biggest innovation was, of course, declaring independence, and otherwise taking the entirely theoretical writings of the Enlightenment and turning them into policy.
This brings us to the Constitution. The Enlightenment was big on ideas, but rather short on specifics. It was very hard to come up with a plan of government that would create a functional national state while still retaining popular support. It was so challenging, in fact, that it took the Founders two tries to get right. The Articles of Confederation, drafted during the American Revolution, created an dysfunctionally weak national government. It could not fund the Continental Army during the war, and could neither pay war debts nor regulate the flow of cash and credit after the war. The looming threat of a financial crisis caused the wealthy and powerful in the various state governments to convene a Constitutional Convention. This was a contentious meeting. Two-thirds of New York's delegation walked out of the convention in protest. Several attendees refused to sign their names to the document, and opposed it in their home states. The Constitution was so controversial because it 1) created a stronger central government that was 2) less democratic than what came before. Delegates had to figure out, for example, how big to make Congressional districts to ensure that no particular constituency could dominate them default. This was mostly guesswork, and it was swimming upstream, intellectually. The Constitution was, in part, the product of the realization that Enlightenment philosophy alone was not enough to build a state. The admission that practicality sometimes had to trump ideological purity was the great intellectual innovation of the Constitutional Convention.
Sources: Bernard Bailyn, The Ideological Origins of the American Revolution
Susan Dunn, French Lightning, American Light
Jack P. Greene, The Constitutional Origins of the American Revolution
Woody Holton, Unruly Americans and the Making of the Constitution