Why is the Euro a bad idea compared to other currencies? Many economists, including Nobel laureates Paul Krugman and Joseph Stiglitz, have said that the Euro was a bad idea from the beginning and is a major reason for all the current economic problems. My general understanding of this issue is that different countries with different monetary policies would have more control over how to control their debt and financial obligations instead of being tied to the same rules that all the other countries have to follow. This makes sense to me.

However why does the USA and China not have similar issues even though they are very large countries with a single currency as well? Should the US and China actually be using different currencies for different regions?
One of the key requirements for a currency area to be successful is that there must be a sustainable political mandate to support fiscal transfers between regions, to provide the balancing mechanism that would otherwise have been performed by fluctuating currency rates between those regions.

Another is that labour can move freely between regions.

The Eurozone, it turns out, met neither of those requirements:

Language barriers have impaired freedom of movement.

And although Northern Europe was happy to reap the benefits of currency union during the good times, it has proven to be unwilling to make the fiscal transfers to the South during the bad times.

Exacerbating this, Greek admission to the Euro zone was created on the back of some fraudulent accounting courtesy of Goldman Sachs. Based on the real numbers, Greece would never have been admitted.