Chatters and people who post on Usenet will often tell others to stop shouting. It's another way of saying, "TURN OFF YOUR CAPS LOCK." When someone is TYPING AN ENTIRE SENTENCE IN ALL CAPITAL LETTERS, that person is SHOUTING. It is not proper netiquette to TYPE IN ALL CAPS, especially in e-mail.
In fact, it's annoying. If you're in a hurry (or want to do what many people online already do), then type in all lowercase-unless you want to imply that YOU ARE SHOUTING!