Here's what seems like a pretty fundamental one: Why do national government's now borrow exclusively from the private sector instead of from their own central banks and as a result pay high interest instead of essentially none, thereby greatly increasing the burden to the public and the national debt?
If central banks are to lend to the government, they would have to print money and that is potentially inflationary. Forward-thinking countries try to control such a temptation by legislating in advance that their central bank be independent. Most modern governments have independent central banks meaning the actions of the central bank are largely insulated from any sort of political pressure from the government. There are shades of grey here; some countries like the U.S. have central banks that are very independent (although the Fed chairman does report to Congress and is appointed by the President) while in countries such as India, not so much. In the U.S., the independent Fed lends to banks but never directly to the US Treasury.