If you have a transit here without korean visa, but have enough time, you still can take a free tour to see some interesting places. You just need to pay 10$ for the security guard.
Try chadol deonjang Chijae at the Byukjae galbi @ the foodcourt 2nd floor if you want to try the real Korean. It is the bean pastesoup with some beef, bit spicy but great.
Convenient place to buy Korean cosmetics that wouldn't be counted into the check-in allowance. Lotte duty free shop offers better deal for some bundle products!
Quite far from Seoul. The best way to get to downtown Seoul is by a bus or with a taxi. Very organized inside with a nice selection of stores. For coffee, go to Paris Baguette. pleasantly surprised!
Free high speed WiFi good for video calling. Clean and organised immigration check points. Queue moved very fast due to highly efficient immigration staffs.
Leave plenty of time to check in, clear security and immigration and take the shuttle train to your gate. For international flights, the whole process can sometimes take an hour.
For duty free shopping use the terminal and shops located just after security only. if you depart from the far terminal with gates starting as 100, you will not be able to return after the train!!!
pick your stuff you bought from Duty Free in Town at Gate 121, make sure you go to the right counter by checking your receipt that you bought from Lotte or Dongwa. No need to waste in the wrong line.
Head upstairs in the departure area past immigration to the free Rest and Relax for free reclining chairs in quiet area (good for nap) and free showers.
Flying to SFO? Check https://foursquare.com/tek/list/free-wifi--power-in-san-francisco for a list of places with free Wifi AND power outlets to work from in SF
At midnight, Asiana lounge is closed. You may get 10,000W coupon for refreshment at airline check-in counter if you are star alliance gold member, but almost all food shops are also closed!!!
Departure from plane to a train that takes you to 'passenger terminal' from where easy info for bus, taxi, cab provided by the ladies at the 'yellow' info counter!