It was revolutionary because it utilized all the economical, technological and political advancements of late 18th century. The evolution of political control (no more feudalism, but centralized regulated government) enabled one seat of power, Paris, to organize planned mass actions of whole populace - from production of uniforms and muskets, to very fighting in the war.

In Medieval Times, yes, every lord could mobilize certain number of peasants if needed, although this was rare. But you can't extrapolate from that, because the whole country couldn't have ability to do the same, because of lack of logistics, but also because of the nature of feudalism - Let me try to explain - Lets say a medieval kingdom has 1 million people living under 10 different dukes, of which every has around 10 counts under him. So let's say every count has 10 thousand people under him. He could muster thousand or so for a fight against neighboring count, scraping a barrel of human resources by arming his own peasants, but he could only send around couple hundred to the army of his duke. Same goes for the duke. So, although all counts separately could gather hundred thousand 'soldiers', the Kingdom itself probably wouldn't be able to gather more than 10-15 thousand soldiers (a large part of them knights, professional archers, etc) combining kings own forces + duke's forces + forces sent by counts.

In 18th century, before French Revolution, armies were professional and standing - Feudalism was practically gone, but Kings still relied on not-so-big armies made up of professionals (either volunteered or 'impressed). Lack of nationalism or general interest in army/serving a king/going far away from your village meant that it was almost impossible to create bigger armies. In the moment of American Revolution, Britain (Which had around 8 million people in G.Britain itself) had only around 48 thousand soldiers everywhere, and by the end of the war, they rose that number up to 100-150 thousand, after painstaking impressment processes that were normally used only for the navy recruitment. And desertion rates were staggering for modern standards, up to 1/4 of soldiers would desert in the first few weeks of service.

Thus, when those countries invaded France during the revolution, they did so with their modest-sized armies. France, in the midst of a revolutionary fervor and first modern example of strong nationalism, and sense of defending not some King but their own country, created a system that was able to mobilize huge numbers of people and even bigger number of people working 'behind the lines' - More than 1.5 million soldiers were mobilized by the end of the process, though it is questionable how many of them were actually armed and uniformed, probably no more than half. This influenced other nations into developing similar systems (England though never really started relying on conscription until WW1), and further advancement of technology and economy allowed for bigger volunteer/conscript armies to emerge during 19th century, like during American Civil War.