Tourism has been their mainstay for over a century.
... & non-payment of taxes!
Basically, Northern Europeans play by the rules & Southern Europeans try to ignore them.
I seriously doubt of a Greek default will affect world markets etc... as everybody has been expecting it for some time.
It's about time they grew up & started paying what the owe though.
A couple of facts that emerged during the start of their problems.
Greece has more owners of Porsche Caymens, than any other Euro state & less than 4% of Athens residents claimed they had a swimming pool (until the government used Google satellite to show them otherwise).
We have all been waiting for Greece to mess up. They have been putting holding measures in place ever since the calamity started, but the people have little stomach for it.
Depending on the effect it could have on the euro. It could affect the entire world finances.