And of course, even the various countries of Western Europe all have MAJOR cultural differences and speak numerous languages. Still, the fact that the people all resembled each other fairly well managed to mitigate this fact when multilingual persons traveled across borders. But what's tripping the EU up is the addition of ten new members, most of which are post-Communist countries with still-depressed economies. Prior to this integration, the EU only had to subsidize the pains of a few small nuissance members like Ireland.
Contrast this with NAFTA. When Mexico's (a huge country comprising about 20-25% of the population of NAFTA) peso collapsed in the mid-1990's, Bill Clinton had to shovel billions of dollars in aid down there to keep their economy from completely imploding, because by then our economies were so integrated that our position depended on theirs. And if it was bad in the 1990's, it's worse now, and it'll be worse still if we break down barriers to Brazil, Argentina, etc.
Yes, NAFTA + FTAA is going to be the mother of all money pits for America if ever enacted. This, at a time when SS/Medicare/Medicaid are going bankrupt and our current budget deficit, which does not include these programs, is in the red to the tune of $450 billion on top of a $7 trillion (and rising) national debt. It's pure insanity.