As much as I enjoy leaving airports in favor of brief city visits, a 6-hour layover does not allow for a lot of time in Seoul. Getting to town (immigration, waiting for the bus, bus trip) may easily take two hours. For the return bus trip, you have to budget another 60-90 minutes. If you want to be back at ICN 60-90 minutes before your flight, you'll probably end up with less than 2 hours in Seoul. I once took the bus to Itaewon on a similarly short layover (to catch a quick lunch with an old friend), but ime it'd be pointless to go through this hassle and stress for some "sightseeing."