The thing is that Japan is actually in a deflationary period. You can easily pay debt with increased money supply in that scenario. Similarly, in Japan the debt is owned by the people of Japan and a big question is whether or not that debt ever comes due. There is an ageing population (which means people will want their money back), but there is also no exemption on inheritance tax. Anybody who dies with money in the post office (which lends all its assets to the government) is essentially forgiving the debt for 30% of that money. Japan has also historically had very low taxes (for example very low value added tax) and this is slowly being corrected. It's placing itself in a better position to pay back debt (at the expense of the economy).

Given that, I'm not really sure that Japan is a good example. The US, as well, is in a weird space. China has been quite keen to buy up US debt. This is propping up the US dollar, which I think is an important strategy for Chinese growth. However, I expect the other shoe to drop at some point. Once the Chinese economy is strong enough that they don't want to be the "cheap manufacturing country for the world" (see Japan and South Korea for other examples of this), I suspect they will drop US debt like a hot potato, favouring strong buying power over strong selling power. If the shift their economy and compete head to head with the US in the "value add" sector, I think that debt may cause significant problems.

But this is complicated stuff and I really don't know what I'm talking about :-) It's fun to speculate, though.