Total debt as percentage of GDP

[King]

I have one question.  I think I know the answer but I want some confirmation:

Which economically significant nations have a debt % of GDP that is low?  As far as I know, the only two major players who aren't running a fairly large total debt are the Chinese and the Saudis; and both China and Saudi Arabia built the foundation of their economy on exports to these debted nations.

Okay, one more question.

Doesn't debt, by rule, only becomes a problem when the collectors show up at the door step?  To me, this seems like it isn't as bad of an issue as it looks on paper--because on a global scale, if any country tried to collect on another one's debt or stopped the loaning process, that country would eventually get screwed over in the chain reaction such a move would start.  Thus, nobody will ever make the move to collect on this debt.

Then again, I'm not an economics professor.  Please correct my logic, wizards.