A Washington Post article on the problems of restructuring of Greece's debt discussed factors that affect country's ability to carry debt. It neglected to mention the issue of whether it borrows in currency it issues. If a country is like the United States or Japan, and borrows almost entirely in its own currency, then it would only default on its debt as a political decision (e.g. it refuses to extend a debt ceiling, authorizing the debt to be paid).
Since it issues its own currency, it can always issue the currency needed to finance its debt. There markets seem to understand this point very well. The countries that issue debt in their own currency (e.g. Sweden, Denmark, the UK) consistently enjoy lower interest rates on their debt than countries with comparable debt burdens who do not have their own currency.