Why is federalism so unique?
Federalism is no longer really unique, at least not in theory. There are many nations that divide power in some way between a central, national government, and smaller, regional governments. But federalism as laid out in the United States Constitution represented a creative solution to a classical problem about republics. It was almost universally believed that a republic was only a suitable government for a very small polity, like the cantons of Switzerland or the city-states of Italy.
Larger republics, like Rome, had failed. So the federalism that the Framers created was designed to create a powerful central government that was deemed essential to governing such a large territory while at the same time maintaining a measure of local autonomy in the states. This would not only more effectively preserve American liberties, it was thought, but would keep the federal government from overreaching into issues that it was not equipped to deal with.
It should be remembered that even in the 1780s, the United States was characterized by a variety of sectional interests. The federalism that the Framers developed, in attempting to balance these interests as well as the powers of government, was unique for its time. Some might argue as well that the specific powers given to state governments, such as education, are unique as well. Another unique aspect might be the continuing debate over the extent of federal power relative to state powers, one which remains very much alive today.