Let's not kid ourselves, there are major, major differences between Catholicism and mainstream American Protestantism. Things Catholics believe that mainstream American protestants don't believe include:
Transubstantiation during the rite of the Eucharist
Papal infallibility
Worshiping of "saints"
Salvation through faith and good works (I'm pretty sure they still believe this, but it's all off the top of my head)
Priests being "normal" people's only direct line to God
Condoms spread AIDS