They have a huge second mortgage exposure that is the most toxic of the toxic assets: http://cfcsux.blogspot.com/2009/04/homeowner-aid-plan-caught-in-second.html
They huge potential losses according to CreditSights. Worse than B of A and Citi!
Many of Wells Fargo's losses are held off balance-sheet (a la Enron) http://www.minyanville.com/articles/print.php?a=20901
I've covered Wells Fargo extensively on this blog - type "Wells Fargo" in the search box at the top of the page to see all my previous articles