Greg Mankiw, chairman of the Council of Economic Advisors under George W. Bush, explains that President Obama's large bank tax is pretty straightforward economics:
One thing we have learned over the past couple years is that Washington is not going to let large financial institutions fail. The bailouts of the past will surely lead people to expect bailouts in the future. Bailouts are a specific type of subsidy--a contingent subsidy, but a subsidy nonetheless.
In the presence of a government subsidy, firms tend to over-expand beyond the point of economic efficiency. In particular, the expectation of a bailout when things go wrong will lead large financial institutions to grow too much and take on too much risk. ...
What to do? We could promise never to bail out financial institutions again. Yet nobody would ever believe us. And when the next financial crisis hits, our past promises would not deter us from doing what seemed expedient at the time.
Alternatively, we can offset the effects of the subsidy with a tax. If well written, the new tax law would counteract the effects of the implicit subsidies from expected future bailouts.