America's been called a "grand experiment". I kinda like that -- I like the empiricist, Enlightenment choice of words.
An experiment -- you start out with something and see how it goes, and you can adjust the parameters as you go.
Regardless of political persuasion, I think most of us can agree that America was founded on some pretty great principles and some pretty awful ones too, and that a couple of hundred years later we've come a long way but we're still very much in the "Pretty good but not perfect" category.
Anyhow. As America's Birthday goes, I gotta go with September 17, 1787, the date the Constitution was ratified. But hell, the Declaration of Independence is pretty neat too, and worth a barbecue and some beers. (American beers. Though I noticed there were some British beers heavily marked down in the store the other day for some reason...)
Oh, and speaking of those two documents, this is on the front page of dictionary.com today:
Great job, reference.com. Great job.