Actually Iceland, like Ireland, had a banking crisis, rather than too many government loans to pay. They just could not possibly bailout their banks (it would have been 10 times GDP), and had to watch most of them go into bankruptcy.
Some people lost the money in their accounts (a lot of Brits), and the Icelandic banks had to be restarted over again.
So part of Greece defaulting would be losses to banks that might sink them. But additionally, they might not get any new loans, and have to sharply cut back to only what they have cash for - which is very little. There are already significant shortages of medicines in Greece. They could literally end up with food shortages, like Venezuela or worse, if they are cutoff from credit and their banks go under. They import a lot of their food.
They would have to deport ALL their illegal immigrants, as they can't afford to keep them on welfare. nor have them take jobs that native Greeks would need.
Then they would have to fire 75% of their government employees, who would have to then get real jobs in the private sector.
I think a lot of blood would flow if they tried either, much less both.