Commercial transactions with other countries would be frozen for some time, and it is not known how long this would last. Shortage of medicines, fuel and food would be more than evident. Let us keep in mind that Greece is not a country that can satisfy its own needs, unlike Argentina, which "feeds" the whole world. No foreign company would be exporting to Greece, while the currency instability existed. Fuel and medicines would be imported with a "dropper" and this would not be done through trade but through intergovernmental agreements.
National Security
Due to the shortage of fuel, but also because of the instability and lack of institutional allies, Greece might turn out to be a bull's-eye for the neighbouring countries, and its sovereignty might be at stake -not in terms of decision making, but in terms of its territorial entirety. A country which is isolated and shaken by social unrest could become an easy target for anyone who wants to challenge its borders. There would also be a serious problem for the police because there would not be enough fuel for police cars.
Social peace
The experience of Argentina suggests that at least in the major urban centres there would be groups of citizens who would try to survive using violence. Dealing with them would be at the east difficult, and perhaps the army would have to take over, whatever that would mean.
Debt
Government debt would increase by the same percentage with which the euro or the dollar would appreciate against the drachma. According to estimates, the debt could reach 200% of the GDP and therefore it would not be possible for it to be serviced. Then Greece would be forced to resort to uncontrolled bankruptcy and a huge debt cut, which in turn means that for decades it would remain outside the international markets and would not be able to borrow.
Tourism
In theory the big winner would be Greek tourism, as prices in the industry would decrease significantly. Nevertheless, social turmoil and instability would be the factors that at least initially would repel foreigners wishing to visit Greece.
Export
The second big profitable industry, again from a purely theoretical point of view, would be exports. Nevertheless, the lack of access to raw materials and the inability to finance the first few years would offset any benefit that might result from the devaluation of the drachma.