Sen. Charles Grassley, R-Iowa, opposed Lew's nomination. He cited questions about his time at Citi, as well as Lew's compensation while working as chief operating officer at New York University.
"Mr. Lew's eagerness and skill in obtaining bonuses, severance payments, housing allowances and other perks raises concerns about whether he appreciates who pays the bills," Grassley said.
One potential weakness for Lew: His relative inexperience with financial markets and international economic crises — areas that had played to Geithner's background. Analysts think Lew will keep pressuring Europe to deal aggressively with its budget and debt issues. But they think this will consume less of his time given that Europe's debt crisis now poses less of a threat to the global economy.
On trade, Lew is expected to keep prodding China. The U.S. trade gap with the world's second-largest economy hit another record high last year. No breakthrough is expected, though.
Lew will also need to calm investors who have grown concerned about possible currency wars after Japan's new government sought to lower the value of the yen as a way to boost exports and its weak economy. A weaker yen makes Japanese goods cheaper overseas and foreign goods costlier in Japan.
And Lew will need to defend the Dodd-Frank Act, which overhauled financial regulation after the 2008 crisis. Since the law was passed in 2010, Wall Street has fought to weaken many of its stricter regulations.
He may also need to work on his signature, which starts off with a soft "J'' but is followed by seven loopy scribbles that render it illegible. The Treasury secretary's signature is emblazoned in the lower right corner of U.S. dollar bills of all denominations.
When he announced Lew's nomination, Obama said Lew had promised to work to make one letter legible "in order not to debase our currency."