It's a start

Krugman:

Private debt... doesn’t directly make us poorer, but it increases our macroeconomic vulnerability.

Yeah. It makes financial crisis possible. It makes debt-deflation possible. It makes bizarre policy responses necessary, so that people question government more. And like that. Macroeconomic vulnerability.

Plus, it increases financial income relative to productive income. This is good for the growth of finance and speculation, and bad for the growth of output.

Excessive private debt is the original source of inadequate growth. Plus, it increases costs, and drives cost-push inflation. And like that.