The world will still operate but economy may have to re-start and that may be a good thing.
It's not the first time that USA has default their USD bond holders. After WW2, anyone holding USD used to be able to convert to gold from the US gov. That's why we call USD in Chinese "Mei Jing" meaning "American Gold". However, in the late 60s to 1970, because of the Vietnam war, US printed so many USD that they can't continue to maintain the USD convertibility to gold that in 1971 President Nixon had to cancel this arrangement and go on a default on all convertibility agreement. This is also known as the "Nixon shock". For details of this you can google it or read it here in the wikipedia page.
http://en.wikipedia.org/wiki/Nixon_shock
So it's not the first time that US default on it's obligations.