What is bad about debt? I guess you have a point that most people seem to take debt for granted as a bad thing without really understanding when it goes bad. We have all heard a little debt is a good thing, but when does it become too much? This looks like a very complex question for the likes of most of us without any clue.
I think the two issues are sustainability and confidence. How long can the US government sustain a compounded annual deficit, and how long will lenders be confident in buying US credit? Besides borrowing and repaying money, the government has a country to support, and a growing and aging population with certain expectations regarding maintaining or increasing their quality of life.
How valid is your assumption that the government can always borrow at near 0% interest (thanks to inflation)?
If the government runs annual deficits, it has to continue to borrow new money every year. At 0% effective return (your assumption), it's not a very attractive investment for lenders. They may start to look at other markets, like China, or emerging markets, rather then injecting more capital in the US. The US will have to raise interest rates to attract lenders, increasing their cost of borrowing. As we've seen recently with the Internet and housing bubbles, the collective loss of confidence is not always a gradual thing.
Also government obligations will start increasing once baby boomers start retiring. They will expect pensions from a system they contributed their working life, and require medical care. If the government does not default on these obligations, they will be required to borrow larger and larger amounts of money.
If our source of borrowing dries up, the government will also be unable to maintain a military, being unable to pay or feed them or provide weapons and protection. This leaves our country vulnerable. Without a strong military, one of our less debt burdened enemies will be able to invade and overtake the USA.
One thing that contributes to debt sustainability is a GDP that continues to increase. What happens when GDP flattens or decreases? (see 2009:
http://en.wikipedia.org/wiki/File:WeltBIPWorldgroupOECDengl.PNG
). In recent history, many of the jobs have been exported to cheaper labor countries like Mexico and China. If we do not restore domestic jobs, private consumption will decrease, forcing more companies to close and layoff employees, potentially creating a vicious negative spiral, and all of it negatively impacting GDP.
Two common concerns about increasing GDP is that it often involves increasing negative side-effects, like depletion of natural resources, or harm to the environment, e.g. pollution, deforestation, etc. For example with oil and gas, we will need to start employing more costly methods to extract enough to meet increasing demands, and these can be more harmful to the environment.
At some point, quality of life starts to suffer for many potential reasons: the retired are poor and sick, without access to medical care; pollution from industry required to support ever increasing GDP making the young and healthy sick; destruction of natural habitats which start to impact things necessary for human survival; increasing global warming (for those who think it exists) will cause greater and greater swings in climate, resulting in larger natural disasters.
If quality of life suffers, or opportunities have all been exported, our children may decide they do not want to repay the unmanageable debt caused by their parents, migrating to other countries with more opportunities, and changing nationality.
I know this answer is all over the place, with many dire predictions of doom and gloom, but basically an increasing debt is only sustainable when GDP continues to grow at the same rate, and that we have an endless supply of lenders willing to supply low-interest loans. I'm not sure either of these assumptions are valid in the long term, and when they are not, just about everything that we take for granted today can start to go wrong.