Much like its cousin, common sense, common courtesy is a bit of a misnomer in that it's really not all that common.
So, I ask you CGB, what irks you the most here? What don't people do anymore? And get off my lawn.
1) Please and Thank You. They've very simple words. But they change so much. If someone does you a favour, thank them. Because it honestly does bother me when you go out of your way for someone... and they can't return even a simple "thanks"
2) Stepping back. Ceding authority or leadership. I know our lives tend to revolve around our "leadership" accomplishments and "taking charge", but... news flash! You're not always going to be the best at anything. If you're working on a group project for a class, maybe another group member can lead in one area and you in another? Or, if you're at a career fair and someone is talking to the recruiter from Goldman Sachs... just let him talk to her before you jump in with your pitch about why you're amazing?
3) Listening. We seldom have discussion anymore. They're not even arguments. They're shouting matches. If you ask someone their opinion and all you can do is come up with counter-arguments as they talk, well, then you're not interested in their opinion; you're interested in validating your own. Perhaps if we listened to one another and made an attempt to understand other points of view, we'd get along better?
Okay, CGB, what are yours?!