The first time I used the Seoul subway, I lost 500 won (US$0.46) because of its stupid ticketing method.
When you get a ticket issued from the machine, you pay a standard fare–about 1,350 won, or more depending on length–but there’s also an additional 500 won highlighted on the screen. If you, like me, want to try to take out the subway machine in its local language, then you will click around on the buttons until you eventually order a ticket, but you won’t comprehend what the extra 500 won is for.
It’s not until the next time that you use the subway machine and you reach into your pocket and find that you still have the old subway card in your pocket and then you try to reload your subway card and find that you can’t, then you click the buttons to buy a ticket, and the 500 won is still on the screen, and the machine issues you a new card, that you finally realize, WTF.
In fact, the 500 won is a deposit on your card, and you have to put the card back in a card return box at the end of your trip to get the refund back. (Since I still had my card in my pocket, I might have ultimately got the refund back. However, who cares?) For someone who has never used a subway, this might seem like a perfectly reasonable system to guarantee that the cards are returned.
But if you have used a subway, you might notice that many systems issue you a token (Nanjing) or a card (Beijing, Shanghai, Hong Kong) and then the turn stalls have a built-in system for you to deposit the card upon exiting.
So, first point: Remember to deposit your card in the refund box at the end of your trip on the Seoul metro.
Overall, however, I have high praise for the Seoul metro. It has very many cars connected and is not crowded like Beijing or Shanghai or Nanjing often are. Its heated seats are highly praised in the winter.