My pal John Carney takes a crack at it:
1) Let’s start with the idea that we’re going to tax banks based on “riskiness.” How on earth do we expect the government to assess this? The government has an absolutely awful track record when it comes to assessing risk. Before the crisis, regulators put in place mandatory capital requirements that they believed were “risk weighted.” The result was the massive over-indulgence in risky mortgage backed securities that almost destroyed the financial system. A risk tax would just result in new pressure for banks to adopt the regulatory view of risk. No thanks.