In my country you are supposed to let people exit from the elevator or trains first.It is thought to be polite. It
is considered impolite to prevent people to go out.
You should not cut someone off. It is thought to be impolite. You should wait till they finish what they are saying.You should
chew with your mouth closed. It is thought to be polite. And you should not
start talking with a mouthful of food.
In my country it is considered polite to treat people nicely even if you don't have to.
Don't talk loudly in public places. It is thought to be impolite.