Sometimes I think it is a sign of our times and how our society has changed over the years. 10 or even 15 years ago everyone smiled at everyone. Many still do, but lots of people wonder what you are up to if you smile at them.
In the same mammer I try to return phone calls, letters, Thank you's, and PM's as soon as possible. Not everyone does...and it makes you feel kind of unimportant.
I like to hold the door for people when given the opportunity. People are often very surprised when I do this. It used to be routine for people to hold the door for others.