I am aware of the fact that many of the political principles discussed in the US founding documents come from John Locke, the British Bill of Rights etc..
Compared to previously established national constitutions, just how different and groundbreaking was the US Constitution? Specifically, I was hoping someone could comment on the foundation of the legitimacy of the US government (ie. the "We the People" concept)
I would not say it was ground breaking. We in the U.S. tend to think our democracy, its ideas, and structure were unique then. As you have mentioned they were not. If you look at the Instrument of Government under Cromwell's short government there are many similarities. Take a look at this website which has the full text of it- http://www.olivercromwell.org/protectorate/protectorate_6.htm
If you want to look at the development of American democratic principles take a look at Jack Rakove's Original Meanings and Edmund Morgan's Inventing the People. Also look at Between Authority and Liberty by Marc Kruman. Its a book about the development of the State Constitutions, I don't think its in publication anymore but there are plenty of used copies.
"We the people...", its a great opening that is very misleading. Who are "the people" in the late 18th Century? Today we consider everyone, men, women, all religions and races. Yet then it was landing owning white men. And the people were whoever the individual states decided who could vote. There are several reasons for this. One concept that permeated through the constitutional period is that of the independent land owner. In this case if you owned land you were not beholden-ed to anyone. Your life was in your own hands. If you rented property you could be influenced by the property owner to vote a certain way. That is why there was so many voting restrictions early on. The writers of the constitution wanted independent minds participating in government.