As a general rule, we worry too much about deficits and debt. Yes, red ink matters, but we should pay more attention to variables such as the overall burden of government spending and the structure of the tax system.
That being said, Greece shows that a nation can experience a crisis if investors no longer trust that a government is capable of "servicing" its debt (i.e., paying interest and principal to people and institutions that hold government bonds).
This doesn't change the fact that Greece's main fiscal problem is too much spending. It simply shows that it's also important to recognize the side-effects of too much spending (if you have a brain tumor, that's your main problem, even if crippling headaches are a side-effect of the tumor).