Greece wasn't a third world country before the EU, and it won't be after the EU. Sure, leaving the EMU will cause a lot of near term pain, but Greeks are used to inflation. They lived with it forever. They have a massive underground economy. They ignore taxes (1/3 of them, by last estimates). In no time, people outside Greece will begin to find cheep holiday deals in Greece and they'll come back because their currency will go a long way. The Greeks will be back on the road to prosperity faster than most think.
People used to talk about how Iceland would pay, too. But they're not all that bad considering they told everyone to go to hell.
The EU can't seize Greek sovereign assets. All they have is a bluff. "Pay us or you're out." But if getting "out" means it's better for Greeks, how is that a threat?