McKinsey Global has a 94 page report on Debt and deleveraging : The Global Credit Bubble and its economic consequences and the Economist magazine has a special feature on debt and created a global debt map.
The sustainability of debt shows that higher economic (GDP) growth after the cost of financing debt indicates an ability to grow and pay for more debt. This is an advantage for high growth countries like China and India and other emerging markets.
Most of the debt growth has been in residential mortgages. There are four main categories of debt (government debt, household debt, financial debt and non-financial debt). Financial debt is debt for banks and other financial institutions and non-financial is for other kinds of companies.
The deleveraging (reduction of debt and leverage) will reduce global GDP growth for 2-3 years.