Our Bill of Rights.
Until it was written most countries had no guarantees of press, assembly, or religious freedom. Even 'enlightened' countries could put people in jail with no due course, could arrest people at will on 'suspicion' and could effectively imprison someone for thoughts that were dangerous to the state.
Our Bill of Rights put in writing, and have been backed up (although some of them took a long damned time) our basic human rights to assemble as we please, worship as we wish, say (politically) what we think and go where we want to go. It's not carte blanche but it works pretty well.
It's not our constitution that has made us successful, you could plug in any of the government forms used by almost any other country and it would probably have worked just fine here. What made us outstanding, and for sure was looked upon by every enlightened person in Europe and America, was our guarantee of the rights of the citizens to reasonably do what they wanted to do without fear of the government punishing them. You have to remember, in 1790, there weren't too many countries where you could do that.
