Reply to All
I've blogged about the pitfalls of e-mail before. But now I want to discuss an e-mail feature that can be quite dangerous, especially in a work setting: the "reply to all" button.
We can all probably think of a time when we abused the "reply to all" button, either on purpose or by accident. When I was the editor in chief of my college newspaper, I accidentally hit "reply to all" when I should have hit "reply" and sent a personal e-mail meant to go to an individual staff member to the entire staff. Even though it was only a mistake and I apologized immediately, I felt horrible.
Not only can "reply to all" be dangerous because it can broadcast private messages to a wider group, but some people may use the button to "stir the pot." Let's say your manager sends an e-mail to the entire staff about a new policy. Sally, who abhors the new policy, decides to "reply to all" and share her views with the entire staff. Five or six other coworkers follow suit. 
First, this unprofessional; if Sally has a problem with the new policy, she should set up a time with the manager to discuss it. It isn't something that should be discussed over e-mail with the entire staff. Second, Sally's probably going to tick off the other employees who are mad that they now have an extra 15 e-mails in their inbox that say "Re: New policy."
So when is it OK to use "reply to all?" Maybe when you are e-mailing your friends about deciding what movie to see Friday night, or maybe if you are discussing a small project with a few coworkers. Remember to think, "Does everyone on this e-mail list care/need to know about what I'm going to say?" And don't stoop so low to use "reply to all" to fry someone in front of an audience. But if you want to praise your whole team for their support or a job well done, feel free to hit that button! Praise in public, criticize in private.