S&P, not Moody's, reduced the U.S. credit rating. The others, including S&P, have the U.S. on negative outlook. The debt crisis in Europe, "safe haven" value of the U.S., and quantitative easing all coincided to help produce that benign outcome. Then, the U.S did not default.
A partial default would very likely be a different matter. It would demonstrate that a commitment to honor the nation's obligations is no longer a binding constraint on its policy makers. Once they cross the threshold of partially defaulting, they will have established a precedent that the U.S. can default. Once the U.S. is no longer viewed as "exceptional" in the area of finance, there will be a risk premium. I suspect it would be modest, but over the long-term, even a 10 or 20 basis point premium can have a large impact.
Also, it should be noted that absent a self-inflicted partial default, the U.S. does not face an imminent debt crisis. In no way am I suggesting such an outcome is likely absent a fiscal consolidation agreement. That's where the deficit hawks get things wrong. The risk is medium-term and beyond. A timely and credible fiscal consolidation agreement gives the nation a transitional period in which it can stabilize and then reduce its debt relative to GDP. The absence of such a program will, in the medium-term and beyond, reduce the nation's flexibility and reduce the transitional period in which it can act to address its fiscal imbalances.