The difference now is the national debt vs. GDP has already surpassed the height of WWII, and that happened back in April. Where does it end? The obvious worry is the US is going to follow in Japan's footsteps.
As long as America can manage debt like Japan, it won't be a big problem. Like Japan, government debt securities are mostly held by domestic and not foreign parties.
Right, QE infinity. What could go wrong?

It may not be a big problem in terms of a currency collapse or something like that. I'm talking more so about Japan's anemic growth over the past few decades.