What is the major difference between the feudal states of the Middle Ages and the emerging national states of the Renaissance?
The major difference between these two types of states was the degree of centralization of government.
During the period of feudalism, governments were very decentralized. There would, of course, be a monarch who ostensibly held power over the entire state. However, these monarchs did not actually govern their entire states. Instead, local control was exerted by various feudal lords. These lords were technically subordinate to the monarch and had to swear allegiance to him/her. However, they exercised essentially complete control over their own areas and often rivaled the monarch for power.
With the coming of Renaissance states, this changed. A move towards centralization occurred. Instead of having these feudal governments, monarchs tried to centralize power to the point where they had national governments set up to administer their entire countries. This did not happen all at once, but that was the trend that made the Renaissance states different from the feudal ones.