I've been participating in mailing lists and Usenet newsgroups for a very long time -- more than 25 years. In that time, I have learned many very good things, and I've read many very stupid posts. I have seen quite innocent comments explode out of proportion into completely unnecessary flamefests that have resulted in years of hurt feelings. Some of this comes from poor communcation skills; some of it comes from language differences; some of it simply comes from differing personalities.

Here is a list of some of the things I've learned that can make a newsgroup or mailing list experience more pleasant for all.

1. It is not necessary to reply to every message, nor even to most messages. Do not think of the group as a chat room; think of it as a daily newspaper that happens to have lots of guest columnists.
2. A message should provide some value. Messages like "Quite right" and "Me, too" and "Probably so" and "Nuff said" do not provide value. It is arguable whether THIS message provides any value...
3. Assume the worst when writing a message. Read every message you write BEFORE you send it as if it had been written by your worst enemy, and see how many things could be misconstrued. Corollary: almost everything can (and will) be misconstrued, usually in the most unflattering way possible.
4. Assume the best when reading a message. Since most people will not heed rule #3, you should search for the positive connotations in most messages, not the negative ones. A person who writes an opinion you happen to disagree with is not your worst enemy, even when they express their stupid opinions very strongly.
5. Every newsgroup and mailing list has its culture. Some groups are inherently friendly to newbies and stupid questions. Some groups react quite violently to them. Some groups silently tolerate top-posting. Some groups react quite violently to it.
6. Watch and observe a new group or list for several days before making your first post. Learn who is helpful, and who are the troublemakers. Learn which kinds of posts receive scorn, and which receive assistance.
7. Usenet is not a real-time medium. Unlike forums and mailing lists, Usenet is not centrally managed. When you submit a newsgroup post, it gets sent ONLY to your news server. From there, it gets forwarded to the next server down the line, whenever those two servers make their next regularly scheduled connection. That server forwards it on to other news servers. There are hundreds of thousands of news servers around the world, and it can take most of a day before your post makes it. And THEN, some human being has to fire up their newsreader, fetch the new messages, and read it. When they respond, their response goes through the same tortured, windy path as your original. Bottom line: do NOT expect two-hour turnaround to your requests for help on your homework.
8. Don't take it personally. Many readers -- especially younger folks -- tend to read assertive or critical messages as if they were aimed directly at them. In virtually every case, this is not true. We've never met; I have no clue whether we would like each other or not, but whether we do or not is irrelevant. Until proven otherwise, we are professional colleagues trying to further our collective knowledge, not children trying to establish social supremacy at the lunch table.
9. No one knows everything, but some people do know more than others -- sometimes a LOT more. There are a surprising number of people on most lists and groups who truly are world-renowned experts in their field. Posts from those people deserve a little extra consideration, especially since they often do not follow rule #3 above. Do not dismiss a controversial opinion without first figuring out whether the speaker deserves consideration. By the same token, do not take every post as recognized fact without first figuring out whether the speaker deserves consideration.
10. In the world of Google, newsgroup and mailing list posts now last forever. Be careful about stating your own personal opinions as if they were facts. By the same token, do not be irritated if someone posts a seemingly trivial correction to your message. Next year, some innocent newbie will go searching for information on the same topic, and it will be much better for him or her if all opinions are represented.