Here’s how financial firms must operate in a post-crash world.

The bank crisis of 2008 isn't over yet, but one lesson is already clear: The banking buck now stops with the taxpayer. The world's governments have provided an effective guarantee for the whole financial system and for a long list of individual institutions. That means that taxpayers—or the governments who represent them—need to rethink the risks that banks should take and rewards that they should expect.

The popular model of the last two decades has been called the utility-casino approach. Vital economic functions, such as managing the payment system, were combined with a financial casino in which banks not only ran the house but gambled with the utility users' money.

That model is dead-or should be. Whatever taxpayer-protected banks should do in the future, financial gambling isn't on the list. It's hard to draw a clear line between investment and speculation, but regulators should make an effort. Something worth doing is worth doing imperfectly.

As for the banking model that will replace the utility-casino, one principle is that balance sheets will need to be stronger. Hank Paulson may be telling banks to "deploy" their new federally provided cash, but regulators are planning for stronger capital ratios, less reliance on short-term funding, and much less off-balance sheet leverage.

How these more financially solid institutions will actually be run is still up in the air. If governments lose focus, many bad old practices might pop up again like weeds. To help keep them away, here are five principles of post-crisis banking.

First, banks need more regulatory attention. To keep market failure away, the financial intermediation trade needs help from above. The strong emotions of participants often cloud their judgment—regulators can be calm. Confidence is crucial—regulators can protect it. Words are cheap, but regulators can ensure that banks are able to keep their financial commitments.