The Bill of Rights. The original Constitution didn't include a bill of rights. The whole debate was whether the Constitution should be ratified or not because it lacked a bill of rights. Because of this debate, James Madison presented the Bill of Rights (which was originally 12 Amendments; only 10 were ratified at that time) after the Constitution itself was ratified. Without that debate, we might not have had a bill of rights for a few decades after the Constitution's ratification (though the adoption of a bill of rights was inevitable).
