Interestingly the economist, Robert Mundell, who was one of the architects of optimum currency union - ended up going against his own theory and supporting the €. Optimal Currency Union (of which he was a main proponent) suggests that the following are required:
1. Labour mobility across the region. This includes physical ability to travel (visas, workers' rights, etc.), lack of cultural barriers to free movement (such as different languages) and institutional arrangements (such as the ability to have pensions transferred throughout the region).
2. Openness with capital mobility and price and wage flexibility across the region. This is so that the market forces of supply and demand automatically distribute money and goods to where they are needed. In practice this does not work perfectly as there is no true wage flexibility. The Eurozone members trade heavily with each other (intra-European trade is greater than international trade), and most recent empirical analyses of the 'euro effect' suggest that the single currency has increased trade by 5 to 15 percent in the euro-zone when compared to trade between non-euro countries.
3. A risk sharing system such as an automatic fiscal transfer mechanism to redistribute money to areas/sectors which have been adversely affected by the first two characteristics. This usually takes the form of taxation redistribution to less developed areas of a country/region. This policy, though theoretically accepted, is politically difficult to implement as the better-off regions rarely give up their revenue easily. Theoretically, Europe has a no-bailout clause in the Stability and Growth Pact, meaning that fiscal transfers are not allowed. During the 2010 Eurozone crisis (relating to government debt), the no-bailout clause was de facto abandoned in April 2010.
4. Participant countries have similar business cycles. When one country experiences a boom or recession, other countries in the union are likely to follow. This allows the shared central bank to promote growth in downturns and to contain inflation in booms. Should countries in a currency union have idiosyncratic business cycles, then optimal monetary policy may diverge and union participants may be made worse off under a joint central bank.
Note points 3 & 4. The Eurozone does not adopt a risk-sharing system a la USA (when Miami ran a deficit the Fed stepped in and rescued it, not so for Greece). As for the business cycle issue, point 4, we all know that the country out of step is Germany. It went into the € at a low ex rate and also with an aggressive export orientated growth strategy (not to mention aggressive Banks looking for lending, when the funds rolled in) - a recipe for failure. If anyone thinks that Greece is the exception take a look at productivity rates across Europe as in this old article:
http://www.theatlantic.com/business/archive/2012/04/the-euro-is-still-doomed-why-most-of-the-news-out-of-europe-doesnt-matter/256230/Mundell seems to have thought that the economics would "sort" the politics, but as many of us know (in particular the USA) - its the politics that needs sorting first.
So....by contravening an old economic theory, Europe got it wrong with the introduction of the € - and let's not be shy... first Greece and then who?
Having said that... it appears that the current spat has crystallised the argument away from economics and towards politics. A left wing Greek government cannot be seen to succeed. Personally I'm interested in who was democratically elected. EU? ECB? IMF? And whose interests should be paramount? Bankers or people?