Just got back from Seoul two weeks ago, pretty awesome experience
The food is unbelievable (and this coming from an italian), Itaewon has some of the best restaurants i've ever been to.
It sucks if you're not used to spicy food though, i don't mind it but i almost had a stroke eating Topokki and that's not even the strongest food i tried there lol
If you are in a restaurant or a bar and nobody shows up (it doesen't happen often 'cause in general service is extremely fast) to serve you or take order don't indulge to shout out loud, that's how it works there and as said above DON'T give any tips outside for taxis
Stand up and offer a seat if you see an elder on the bus or metro without one
Smoking is extremely cheap, just be aware that in a lot of places smoking is forbidden and people can get really angry if you're smoking when there's children around
You MUST visit all the palaces or what they're called, the weather was not the best for me and i just can't imagine how beautiful they could get in a snowy season
...avoid music shops if you're not into K-pop. in 10 days i only saw like two stores with other kind of music or indie shops, pretty shitty how obsessed they are with idols and such
Ps Korean (expecially Seoul citizen) might seem a little cold in the approach and such at first, but just give it a couple of days, you'll have a great time, i'm sure of it
PPs 90% of shops and cafees has free wi-fi (psw is on the receipts or written at the counter) but if you just want to be sure to always have wifi available take a look here
http://first.wifi.olleh.com/en/index_new.html