He goes through how Citigroup's giant balance sheet was not in fact run aground by $43 billion in mortgage-related assets, but by the fundamental risk any banks face: deposits (money borrowed from us) must be accounted for as liabilities, while assets (money loaned to others) cannot be valued at their future pay-off, but assessed a risk discount to create an actual asset value.
That risk factor has changed, lowering the value of assets and thus the value and stability of financial institutions.
A tip of the hat to James Surowiecki at the New Yorker blog The Balance Sheet. In The World Turned Upside Down, he posted what he thinks are the two most valuable paragraphs. I think the risk discussion is more important, personally.