Your credit rating determines the cost of borrowing money - the AAA status is essential if you want to borrow money (i.e. issue bonds to) some of the biggest, most risk-averse funds in the world. If the US loses its AAA rating then you cannot sell bonds so easily and therefore it becomes harder to raise money or balance the books without politically-unpopular tax raises or service cuts.

Does the credit rating have a bigger impact for huge countries such as the US? I see lots of economically stable countries with poor ratings on the list, including Poland (A-) and South Korea (A).