One of my pet peeves:
In a public setting, the person walking ahead of me allows a door to slam in my face.
What the heck?
How long does it take to glance back to make sure that no one is behind you? And when the person behind you is near, how much effort does it take to hold the door for him or her?
For your courtesy, you might even get a smile or "thank you." Even if the other person doesn't acknowledge your courtesy, you will have known that you did the polite thing (Bragging rights here).
By observing door behavior, you can tell a lot about people and their basic characteristics; a person who thinks nothing of allowing a door to slam in your face is not likely to care much about anything except him or herself.
Another pet peeve:
People who stand in doorways, blocking passersby; if you're going to engage in a long, drawn-out conversation, please move it to a less traffic area, like a coffee shop, bar, park, or low traffic area.
People who insist on blocking passing traffic always seem to act as if it were your fault that their conversation has been so rudely interrupted by your wanting to get from point A to point B.
So, my dear readers, holding doors open for strangers and keeping doorways free of human doorstops are simple things that you can do to help grease the wheels of polite society.