Via email, this is from David H. Levey. He was the Managing Director, Sovereign Ratings, Moody's Investors Service, from 1985-2004:
The recent S&P downgrade of the credit rating of US Treasury bonds is unwarranted for the following reasons:
1) The US dollar remains the dominant global currency and no viable competitor is on the horizon. The euro is heading into dangerous and uncharted waters while deep and difficult political, economic and financial reforms will be required before the renminbi could become fully convertible for capital flows and Chinese government bonds a safe reserve asset.
2) US Treasury bills and bonds, along with government-guaranteed bonds and highly-rated corporates, will for the foreseeable future remain the assets of choice for global investors seeking a "safe haven", due to the unparalleled institutional strength, depth and liquidity of the market. Although there are several advanced Aaa-rated OECD countries with lower debt ratios and better fiscal outlooks than the US, their markets are generally too small to play that role. Since ratings are intended to function as a market signal, it makes little sense to implicitly suggest to investors seeking "risk-free" reserve assets that they reallocate their portfolios toward these relatively illiquid markets.
3) Despite the above positive factors for the US, it is certainly the case that the US long-term debt outlook is deteriorating under the pressure of rising entitlement costs and an inefficient, distortionary tax system. Failure to reverse that trajectory would eventually make a downgrade unavoidable. But the recent discussions signal to me that -- finally -- public awareness of the fiscal crisis is growing and beginning to influence Washington. There is still a window of time -- perhaps as much as a decade --within which structural reforms to spending programs and the tax system could reverse the negative debt trajectory.
4) The bottom line is that the global role of the dollar and the central position of US bond markets make somewhat elevated debt ratios more compatible with a Aaa rating than is the case for other countries, another version of the US's "exorbitant privilege". But that extra leeway is finite and serious reforms to entitlement programs, particularly Medicare, must be made in a reasonable time horizon. If not, global investors will eventually conclude that our political system is incapable of making the needed changes and turn away from US assets, regardless of the institutional strengths of US markets.
[See also the comments by Rajiv Sethi.]