Article I of the U.S. Constitution outlines the powers of the Legislative Branch, otherwise known as the peoples' branch.  It stipulates what the Congress can and cannot do.  However, with regard to your question, it is the longest and most detailed of the three branches of government.  Interestingly, Article II The Executive Branch is the shorter in length and almost abruptly brief.  It is a matter of fact that the framers intended just that with regard to Articles I and II respectivly.  Using the practical aspect of length to enhance their perspective was in fact done quite deliberately.  The power would lie with the people which resided in Article I.  It was written that way to ensure that their point was well understood.  The framers understood what they were doing.

In addition to separating power among the 3 branches and giving the most number and detail of power to Congress, what is even more telling of the framers' intent are the number checks it gave to Congress over the other 2 branches. Congress has powers over the President- Override power, the Spending Power, the Impeachment Power- to keep the President from abusing his position. It also has significant checks on the Supreme Court- the Confirmation Power, the power to create/abolish court systems, the Jurisdictional Power, the ability to set dates/times for the courts to sit, the Impeachment Power. Thus, the powers delineated to Congress for functioning as a legislative body, but also its presidential and judicial oversight tasks, clearly shows the framers' concerns of a tyrannical government and their belief that Congress would most closely reflect the will of the people.

Article I of the Constitution deals with the powers extended to the Legislative Branch, while the second Article deals with the powers to the Executive Branch.  Article one is much lengthier and detailed than Article two.  I believe that this tells us that the framers intended for the legislative branch to be more vital than the office of the executive.  This is not to say that the latter is not important.  The framers were all too aware of the results in a government that does not feature limited and divided power that is articulated in a clear and distinct manner.  This was their experience with the British, and to avoid it, the framers designed the articles of government to declare stated powers of the three branches.  Having said that, the framers understood that to consolidate power into one branch moves a nation closer to despotism and tyranny.  Yet, at the same time, having too much power spread in a diluted manner will result in an ineffective government.  The answer for the framers was in a limited government, predicated on checks and balances.

Yet, I think that with Article I's detail and length, it indicates that the framers did intend for the legislative branch to be the seat for the new American government.  This might be due to the fact that the framers admired the Roman Republic's Senate concept.  It might also be a result of the framer's fear of intrusive government; it is much easier for one person (President) to abuse power than for 535 individuals to abuse power (House + Senate).   In distributing the power amongst many individuals, there is a lesser likelihood of power being abused.