Don't say anything that you don't want the whole class to hear. This relates to the previous point. Be aware that your audience sees only the words on the screen, without the voice tone and body language which usually accompany words in conversation. That's why sarcasm is sometimes misinterpreted, because the reader has no verbal clues to tell her that you weren't being serious. So to prevent the reader from taking an innocent comment the wrong way, you should scan your message before you send it to see if any statement you made could be misunderstood.
Most people consider it obnoxious.
If you want to take advantage of a discussion group or professional listserv to find out information for a paper or project, just remember that these are often serious scholarly forums. Students have been known to jump right into the middle of a high-level discussion about Shakespeare, asking for a plot summary of Hamlet. And they wonder why they get flamed. Some also have archived FAQ files to answer routine housekeeping matters, such as how to unsubscribe to the list, etc. Emoticons are little faces which you have to look at sideways.