The bad acting of the American characters was the only thing that really made me wince. Korean filmmakers probably don't usually have the budget to fly a big name (or even a C-lister) over from Hollywood, and I doubt there's a large stable of American actors hanging around in Korea waiting for parts, so I found myself wondering if they just sent someone out to grab Americans off the streets of Seoul. "Hey? Want to play an American in a Korean movie?"
It wasn't entirely the actors' fault -- I could actually tell when Korean conversational idioms had been translated into English for their parts. They must have had fluent English speakers check the script, because the Americans' lines were grammatically correct, but when an American spoke, it just wasn't quite idiomatic in the way a native speaker would naturally talk.
I'm sure speakers of other languages get the same vibe when Hollywood has someone speaking their language in an American film.
Likewise, it's usually easy to tell in British dramas when they're having a British actor play an American. Besides the fact that Americans in British dramas always find some reason to bring up the Second Amendment, no matter how irrelevant to the plot, British actors trying to speak with an American accent sound like they are talking around a mouthful of oatmeal. (Hugh Laurie excepted.) Yes, I'm sure most Americans trying to do a British accent are equally painful.
I was thinking about this all through Torchwood. (I just finished Season Two.) I was undecided about John Barrowman. Yes, he sounds American, but was he really American, or was he a British actor who does a really good American accent? There was just something not quite 100% Yank in his idiolect, I thought. So I finally consulted his Wikipedia entry. Aha! He's Scottish-born, but raised in the U.S., and still uses a native Glaswegian accent with his family.