Retirement savings, stock options, real estate, and any sort of investment account makes your U.S. tax return go from "a 1040" to a towering mountain of special forms. A lot of them also lose you the right to e-file, for some reason. And many aren't supported by tax software, or are handled incorrectly.
A lot of "foreign" banks – i.e. your _local_ banks – won't even talk to you. They tell you to make your investments in the U.S. instead, so you get to pay international wire transfer fees and foreign currency exchange fees on every investment. And if you want to move it back, you get to pay them again.
You won't necessarily owe any tax in the U.S., but you sure pay for it in either filing time (10s to 100s of hours) or expat tax accountant fees.