OK, let's look at structural deficit, GDP growth, and Net International Investment Position.
NIIP is interesting. Despite the US's huge debt, they have a reasonable NIIP of about -25% (I think). So the money is mostly there. It's mostly in private hands, but as long as the private sector is OK, the government will keep collecting taxes.
The obvious problems with the US are its health system (more public funding than countries with free health, but terrible value for money), its military deployments (the military can be an OK if they use it to drive high-tech growth, but actually deploying it very expensive), its prison system (they tend to lock criminals up for a little too long, and they have way too much crime for other reasons).
