How was the American Revolution revolutionary?
Prior to the American Revolution, no colony of a European nation had ever attempted to become an independent state.
From the time that the Spanish first staked their claim in the New World after Columbus' exploration, many European powers had carved up the Western Hemisphere into colonies.
As the population of the English colonies on the East Coast grew, so did their dissatisfaction with the way the English crown was governing them. It took a great leap of imagination for Americans of that time to envision a large independent nation, which is why the colonies originally viewed themselves as separate 'states.'
Only by sending representatives to various 'congresses' and setting up 'committees of correspondence' to share news among them were they able to take that step of viewing issues as affecting all of the states, not just their individual one. Forging a truly "American" national identity would take a long time, in fact, some historians don't believe that really took place until after the American victory in the War of 1812.
Because the act of breaking away was seen as revolutionary, the Declaration of Independence was written was to justify this action in European eyes. Our Founders knew that we would need help from other European countries if we were going to be successful, and we had to make a very good case before the world to show that the British government left us no choice.