The kind of people who say that the US's debt is totally going to be a problem any year now have generally being saying that for years, as well as saying that about Japan's since the early 90's.
But MMT proposes that a monetarily sovereign currency issuer (like the US, UK, Japan, Australia, New Zealand, etc. but not Euro-using countries, or anybody with a gold standard, fixed exchange rates etc.) actually have the ability to service any debt denominated in their own currencies, and analysis to pretty confidently say that it would not be inflationary to just pay them out (i.e by money creation). The inflation risk was at the time when the spending of the bond money happened, so paying them out with new money is asset neutral - the only difference is it just gives the bond-holder a more liquid asset.
Then there are other useful tools like sectoral balances, etc. that help explain the links between one of the things that really matters (private debt) and how it relates to Government spending and trade surplus/deficit. A few of the twelve or so people who accurately described the great recession a few years before it happened used this method at the core (e.g. Keen, Godley).
It flies pretty hard in the face of conventional economic wisdom, but I haven't yet found an economic theory that holds up so strongly to real world observation, i.e. in explaining why Greece has huge debt problems but not Germany (Eurozone, but trade surplus) or the US (monetarily sovereign), and why Venezuela and Zimbwabe have their problems (debt denominated in foreign currencies, supply shocks, etc.) vs. Japan (monetarily sovereign) while they seem to do similar things, etc.