One major difference was the social composition of the respective revolutionaries. In America, the leaders of the revolution tended to be men of property, men who had a stake in the stability of society. Though ultimate sovereignty resided with the British Crown, the colonists still enjoyed a fair degree of autonomy in relation to their own affairs. Over time, the American colonists developed a sophisticated understanding of liberty, one that they came to believe was under serious threat from continued British rule.
The French revolutionaries, on the other hand, tended to come from a social class that had been systematically denied any serious role in government or administration. The vast majority came from the so-called Third Estate, those Frenchmen not part of either the aristocracy or the Church. They were the ones who paid the taxes, yet didn't receive any kind of political representation in return. Grievances over the patently unjust system of taxation were one of the main contributory causes of the Revolution, and led directly to the Third Estate's representatives unilaterally constituting themselves as the National Assembly.
The understanding of liberty in America was therefore based on lived experience, practice over time. In France, by contrast, the concept of liberty was abstract as those clamoring for its realization had never before had any direct experience of what it meant in practice. This helps to explain why the Revolution in France, as opposed to America, took a decidedly violent turn, and quickly. Liberty had no deep roots within the wider culture, and as such became a major bone of contention between rival factions and groups. As no single group could articulate a concept of liberty acceptable to everyone, it was inevitable that conflict would arise and that the revolutionaries would end up fighting with each other over their respective visions of France's future.
At the time of the American and French Revolutions, the United States was a colony of Britain, while France was a monarchy. Thus, while the American Revolution took place in a colony, the French Revolution happened within the territorial boundaries of the kingdom.
The intentions of the American Revolution were to achieve independence and break away from British control. On the other hand, the French Revolution was aimed at overthrowing the monarchy and changing the system of governance. Thus, while the French revolutionaries wanted to institute fundamental changes to the administration, the American Revolution was a war waged by the colonists to form a new and separate government and country.
The American Revolution started with brief skirmishes between American rebels and British troops as was the case in Lexington. On the other hand, the French Revolution began with a bloody siege of the Bastille that resulted in more than one hundred people dead and more wounded.
The American Revolution was occasioned by the Declaration of Independence by the colonists, while the French Revolution was characterized by the Declaration of the Rights of Man. The Declaration of Independence sought to recognize individual rights and equality, while the Declaration of the Rights of Man sought to end political and social imbalance.
The two most important differences between these two revolutions are that the French Revolution was more radical and, in a sense, more violent.
First, the French Revolution was much more violent than the American Revolution. It is true, of course, that the French Revolution did not lead to a major war within France the way that the American Revolution led to a major war within the American colonies. Nonetheless, the French Revolution was more brutal in many ways. In the American Revolution, most of the killing was military. This is less brutal, at least arguably, than the kind of killing that went on in the French Revolution. All of the executions during the Reign of Terror made the French Revolution much more brutal for society as a whole than the American Revolution was.
Second, the French Revolution was much more radical than the American Revolution. The American Revolution was not aiming for a fundamental change in the structure of society. After the Revolution, much was the same in the United States as it had been in the colonies. The same class of people generally held positions of power. The system of government was similar to what it had been. By contrast, the French Revolution tried to completely remake society. There were no attempts in the American Revolution to create a new secular church or to give new names to months and days. There was no attempt to completely cast down a whole class of people from power and replace them with another class.
Thus, the American Revolution was much less violent in a way and much less radical than the French Revolution.