I just thought it would be interesting for you if I made a list of businesses that are popular in both South Korea and USA.
Dunkin Donuts (It's the "McDonald's" of Korea... you can always find one. But what's interesting is that you don't really see people there early in the morning. Donuts aren't a morning food in Korea - they're more of an afternoon or night time pastry snack.)
Baskin Robins/31 flavors (If you see a Dunkin Donuts, there's a 60% chance you'll see a Baskin Robins nearby. It's so popular! And yes... it's just as if not more expensive. But, comparatively speaking, Korean people tend to get quite a bit more crazy with their icecream as they do with their pizza. For example... when I was doing Hapkido, somebody brought in icecream and it was the craziest icecream I'd ever eaten!!! It had everything of everything in it! It was all colors speckled with all colors: pink, blue, brown, white, red... there were blueberries in it, cherries in it, fudge, nerds, pop rocks... everything. I never knew what the next bite would taste like - and I half expected to find kimchi or chicken in it. I didn't, and the icecream wasn't bad - just crazy.)
McDonald's (yes, all the same meals you hate to love. You'll find the Big Mac, Sausage and Egg McMuffin, Big Breakfast, Pancakes - the McNorm)
KFC (Kentucky Fried Chicken - it's not HALF as good here as it is in the USA).
Burger King (not nearly as frequently seen - but the same menu choices, from the whopper to onion rings.)
Starbucks (duh. And the coffee is MORE expensive. I haven't been there yet.)
Subway (YESSSSSSS!!!! The same goodness of Subway, here in Korea.)
Papa John's Pizza
T.G.I. Friday's (the food here is MUCH worse than in the US.)
Bennigan's (the food here is MUCH better than in the US.)
Outback Steakhouse (haven't been there yet, but I see them around and I do plan on visiting on the right occasion.)
That's all I can think of from the top of my mind - so I'll come back and add more if I remember more in the future. If you want to ask me if I've seen a particular restaurant or business in Korea, just ask! But I have not seen the following common restaurants:
No Taco Bell