Warren Buffett's Berkshire Hathaway (BRK.A -0.04%) (BRK.B -0.15%) owns around $40 billion worth of American Express (AXP -0.56%) stock. American Express' market cap is around $220 billion, so Buffett is a very big shareholder.Given these two facts, you might think that the Oracle of Omaha is expecting big gains from American Express. Probably not -- and that's a big reason why you probably shouldn't buy American Express just because he owns the financial giant.What does American Express do?From a business s ...