I think it would be a mix of good and bad, for both the USA and the rest of the world.
Militarily, the USA would be able to scale back, so it may save us money. On the other hand, not having troops spread across the world would give the USA less "interest" in international politics. Iraq and Aghanistan would be the first to collapse. I am not sure what would happen in the Korean peninsula and beyond. In the Balkans, the USA makes up a large portion of peacekeepers, but so do many countries. Immediately, there would be much more instability in all of these countries, and even chaos, but eventually stability would occur. The question is would the governments that add that stability be good or bad to the people.
As far as debts are concerned, if the USA called in all of its debts, I strongly doubt it would see them all paid off. Likewise, that goes two ways, and the USA would need to pay off its own debts as well. I may be wrong, but I am under the impression that most of the USA debt is domestic, so that wouldn't conflict with isolationism. The real question would be about how realistic it is for the USA to call in debts of others, and what to do if those in dept refuse to pay off immediately. Without any involvement in internation affairs, there would be no way to force cooperation.
Foreign programs, for the most part, would hurt the rest of the world if they were stopped. On the other hand, they would decrease US interests worldwide and slightly decrease US vulnerability. Also, one would expect foreign programs to create US support through PR, but how effective they are in debatable. I think the USA would ultimately see some negative effects though at the same time save some money by scaling back those programs.
What really is the question here, is a scale of two different political philosophies: Imperialism, Isolationism, and a scale between them. Ever since the World War II and the Cold War, the USA has become more Imperialist (by which I simply mean more involvment in foreign affairs, not Empire-building), where up until the first World War (I may be off here) they were more isolationist.
Personally, I think it matters who the competition is. Today, there are no Axis powers threatening to take over the world and threatening our own security. There is no USSR doing likewise while pointing missiles at the USA.
At this point in our history, I think that there are no major global super-threats to our security. Bands of criminals seem to be the only threat to our domestic security. In my opinion, I think the USA should move towards a more isolationist foreign policy, being involved ONLY when it is absolutely necessary. It isn't the type of thing that can be done immediately, but as a scaled approach over years.
...but those are just my thought