As with any game, golfers should think about safety. Golf clubs and golf balls can be dangerous if they aren't used with a little bit of common sense. Don't swing your club when someone is walking around and, conversely, don't walk around when someone is swinging a club. Another rule of golf etiquette, which considering the importance of concentration on the course should go without saying, is keeping quiet. Congratulating a member of your group on a good shot is fine but remember that others are playing.
People are so into their own game they tend to forget there are people on the course.
It can be very distracting to have someone yell out in the middle of your backswing. Besides safety and quiet, golfers should also be aware of the pace of play. People who have just started golfing sometimes slow down those who have a little more experience. But on a crowded Saturday or Sunday, let faster players play through. Or, better yet, play on a day when the course is less crowded.