The two most important political innovations in the Constitutional Convention were the development of federalism and the system of checks and balances. Each of these was an attempt to place constraints on what most of the leaders of the convention believed should be an immeasurably stronger national government than the one that existed under the Articles of Confederation.
Federalism was an attempt to balance the need for a more powerful central government with the desire on the part of states to maintain some degree of self-governance. This was accomplished by first establishing that the federal government was supreme, which was done in the so-called "supremacy" clause to the Constitution. Many of the Framers thought that limiting the powers that were delegated to the federal government would be sufficient as a safeguard for state powers. The Tenth Amendment, added in 1791, reflected the anxieties of some Americans that the federal government would arrogate too many powers. It provided that all powers not specifically granted to the national government should be reserved to the states. While the debate over how much power the national and state governments should have continues to this day, the system of federalism established by the Framers was truly a political innovation. It allowed the federal government to work in a large geographical area without becoming tyrannical.
The system of checks and balances was also an innovation, one not really comparable to any contemporary governments. The idea was that by giving certain powers to each branch, including the power to restrain each other's activities, the Constitution would keep any one branch from dominating the others. The veto power, for example, was vested in the President and was intended to restrain the power of Congress. But Congress could override the presidential veto and had the power to impeach the chief executive. Additionally, only Congress had the power to declare war. These were intended to be safeguards against Presidential tyranny. Over time, the authority of the federal courts to declare laws and executive actions unconstitutional became a major check on both the legislative and executive branches, and the power of the President to appoint justices (with Congressional approval) limited the power of the judiciary, which was not even fully fleshed out by the Constitution.
These two innovations made the Constitution unique from any other plan of government established in human history.