Michael Pettis has an excellent short essay on this point, here is one (scary) excerpt:
A debt crisis must be resolved quickly because there is a self-reinforcing component within the process that can be extraordinarily harmful. High levels of sovereign debt create uncertainty about how the costs of resolving the debt will ultimately be assigned. This uncertainty causes growth to slow by adversely changing the behavior of a wide variety of stakeholders in the economy (as I will describe later). As the economy slows, contingent liabilities within the banking system rise, tax revenues decline and fiscal expenditures rise, all of which push up sovereign debt levels even further and increase both the cost of resolving the debt and the uncertainty about how the costs will be assigned. The consequence of this self-reinforcing deterioration in the sovereign balance sheet is, at first, a slow grinding away of the economy until the market reaches some point, after which the process accelerates and debt can spiral out of control.
Hat tip goes to the ever-excellent The Browser.