What are the threats to European banks?

Investors obviously have a very negative view of European banks. But what are the real threats to these banks? Given the massive government support and public guarantees on loans, the increase in household and corporate defaults at the expense of banks is likely to be limited; Given the ECB's massive interventions, there is unlikely to be any euro-zone sovereign debt crisis that would spread to the banks through their government bond holdings; Curiously, perhaps the greatest risk comes from the correlation with the situation of US banks, which will be heavily affected by the rise in household defaults on loans due to rising unemployment.