It makes The US look like it's made up of nothing but greedy/irresponsible assholes.
This is true. But there's a bit more to it than that.
I'm not hugely into economic history yet (I will be some day ideally) but as far as I know, this is the first time one of the world's major debtor nations has had its rating
changed, not necesserally just cut.
As I said, the US is the world's biggest debtor in absolute terms (in relative terms - like debt to GDP, I think Japan might be), so they basically owe money to the world.
US treasuries are the world's risk free asset; as in, you buy treasuries cos you know you are gonna get your money back. Kind of like when I go to a McDonalds (which has been a while now I think of it); sure, it isn't gonna be the most fantastic meal, but I know it's going to give me value for money, is cheap, and is always available (the secret of the universe is between the T-bills). A cut in the rating, ceteris paribus, means investors will demand a higher yield. Seeing as pretty much every other asset is priced based on their risk relative a treasury, the potential ripple effect is pretty huge.
And finally, the US dollar is the world's reserve currency. This now
must be called into question, as any threat to the ability for people to transact in US dollars due to circumstances implied by a lower-than-AAA rating should be a cause for panic. Fortunately at this stage, things are being held together.
We enter uncharted waters on Monday - my call is a bloodbath, followed by the realisation that nothings really changed. Ofcourse, the alternative is armaggedon.