First, people will be not that friendly as you think. It will be not like Americans who really friendly to strangers. However, do not just judge on them with it. When you know them and get little bit closer with them, they will become one of your best friends. They will run to you when you are in trouble, they will always stay at your side, and consider your problems as their problems.
Second, do not surprise when Koreans drink a lot of alcohol. Drinking alcohol has becoming part of Korean culture. At first, you might feel little awkward but when you just get into it. You will see a world. We have tons of different games and tricks related to alcohol.
Lastly, this came from my experience, do not think Koreans are always angry. When you go to Korea, you will see people walking down the streets with grumpy faces along with their fast walking. They are not angry, but just the little stress from their work. We always have exact place to go, and because of Koreans unique characteristic: doing things really fast, we could be always seem busy.