Rekrul's right about Apgugeong and Kangnam and those places - that's where it's at. I went to the Circle club once and it was great, except I was stuck with a bunch of Korean American girls who didn't know how to have fun.
Itaewon is a fun place to go if you've already been in Seoul for a while because it's just shocking at that point to walk down the street and see so many non-Asian people. There's a bar there that has all this mahogany wood and looks exactly like a stereotypical New England old money sort of place ... I walked in and felt like I'd been transported back home instantly. Itaewon is also a good place to go if you're sick of Korean food and want a (really expensive) burrito or something.
That being said, I'd say the variety and quality of Korean food was definitely something that kept me going. Galbi is amazing, don't get me wrong, but there are so many other interesting dishes you can try, and even some good seafood ones. Insadong is a good place for that because there are all these super-traditional places that serve more unusual food. There are just so many delicious things to eat, and some of them are just as expensive as galbi, so why limit yourself?
The StarCraft matches are the best entertainment money can't buy (because they're free). The huge LCD screens, the hokey lighting and fog machines, the high school girls screaming ... you just can't find that anywhere else. Then afterwards you can chill outside the stadium and chat with the pro gamers and see them interacting with their fans and receiving gifts - it's great to realize that the faces you've seen in the VODs actually belong to normal human beings and not stuck-up demigods.
If you find a girl you like and want to do something a little quieter and more romantic together, there's this very artfully designed river that starts in the city center which is shielded from much of the noise and traffic of the city and is really a beautiful place to walk at night. Definitely one of my favorite parts of Seoul.
Next time I'm back there I'll have some money in the bank and eat lots more galbi and check out the bars and clubs rekrul talked about. And yeah, the white old-enough-to-be-sketchy English teacher guys are fucking pathetic.