Please go and check out the Oatmeal's cartoon If you do this in an E-mail I hate you. Funny stuff and soooo true.
My pet peeve is the one where people inappropriately hit "Reply All". Sometimes it's just annoying, like when a person replies just to say "Thanks" or "Okay" and their one-word reply goes out to everyone on the mailing list. So unnecessary. But I have also witnessed incidents where the person hits "Reply All" and proceeds to discuss something which really should have been kept private between the sender and the one recipient, causing awkwardness and embarrassment all around. An example is where a friend of mine who is president of an organization sent out a newsletter to their entire membership base, and then the treasurer hit "Reply All" and proceeded to chew out my friend about the budget... something which should have most certainly been confined to at least just the executive, not the 100 members on the newsletter list. Oy. Please people, think before you click that button.