Yesterday was Constitution Day and I let it slip by without saying anything profound. Naughty Qwertz. Very bad.
The Constitution's brilliance (aside from the revolutionary idea that government power is derived only from the people) is in its attempt to insulate itself against corruption. The Framers separated government power among the Federal branches, and between the Federal and State governments, so that tyranny could grow only slowly. The Framers knew they were drafting a document composed largely of compromises, and probably knew that would come back to bite the country in the backside at some point in the future. So they created the Separation of Powers doctrine to help slow that down.
Today, we see all sorts of erosions of the Separation of Powers doctrine: mostly the Executive encroaching on the Legislature or the Legislature encroaching on the Judiciary. Of the three branches, the Judiciary tends to be the least "activist" in breaking down the Separation of Powers (though they are not guiltless in this). With the degradation in the separation of powers, the Federal government will become more nimble and we will see changes occurring at an increased pace. The Federal government is not so much a hulking, lumbering, clumsy sloth by design any more as it is by simple bloat.
Unfortunately, the Separation of Powers doctrine also protected those flaws in the Constitution that were born of compromise. Slavery could have been abolished under the Constitution in 1808, but it took 60 more years and over 600,000 dead before that happened. Changing something so fundamentally bad as Congress' power to lay and collect taxes (Art. I, § 8, cl. 1), the Commerce Clause (Art. 1, § 8, cl. 3), the post office (Art. 1, § 8, cl. 7), or changing the ability of something like Amendments XVIII and XXI to even occur in the first place, or repealing bad alterations, like Amend. XVI or Amend. XVII, are all hindered by the Separation of Powers doctrine, not so much by virtue of the difficulty of altering the actual text of the Constitution as by virtue of the difficulty faced by the Judiciary in navigating Executively- and Legislatively-entrenched obstacles to reach a just solution.
Nevertheless, the dispersion of power effected by the Constitution is masterful, and a testament to the brilliance of the Framers.