As far as I can tell, not too much.
Caveat first: I am not an expert in financial instruments.
Still, every major news organization on earth is trumpeting the fall of the US credit rating from AAA to AA+, and I honestly don’t think it’ll have much impact.
In 1998, Japan lost its AAA rating. In January, S&P downgraded Japan to AA-, the fourth-highest rating in the same range as China. There are further warnings that it might fall further.
Yes, there are plenty of reasons why Japan is different — high national savings, high foreign reserves, and low percentage of debt in foreign hands.
So, unlike with you or me, Japan’s credit rating has almost no impact on its ability to borrow money. In fact, it can borrow at substantially better rates than even some well-regarded AAA governments, like Germany (2.3%) or Sweden (2.9%). (See this helpful chart from the NYT for more comparisons.)
I’m not saying that the markets will shrug at America’s debt rating. But the ratings agencies don’t seem to have a huge impact on the kind of borrowing that countries can do. China funded its massive (and mostly successful) stimulus by selling bonds. And the markets continue to rate countries by buying and selling, without regard for what ratings agencies have decided.
The biggest impact will be 1) organizations that are only allowed to hold AAA debt (a stipulation long since made obsolete by the subprime crisis), and 2) the reaction of investors and individuals to this news. If news organizations engage in hysterical hand-wringing, it will make people more panicky. If, on the other hand, news organizations consider this information analytically, it will be less news-worthy, but more correct.