Please note that the original is heavily Wikilinked. Thus a large number of other documents are incorporated by reference.
(*) Editor forced to remove illustration of vandalism due to technical issues.
1. BE BOLD in updating pages. Go ahead, it's a wiki!
2. Ignore all rules, including this one. Don't worry! If you just want to add some useful information to an article in a commonsense way, DO SO. On the other hand, if someone suggests that there is an established and sensible way to do something, please ignore this rule and listen to them.
3. Neutral point of view (NPOV). Try to write from an as neutral point of view as possible. This is a fundamental principle of Wikipedia, which allows us to make a fair representation of the world around us.
4. (But) When in doubt, take it to talk. We have all the time in the world. Mutual respect is the guiding behavioural principle of Wikipedia and, although everyone knows that their writing may be edited mercilessly, it is easier to accept changes if the reasons for them are understood. If you discuss changes on the article's talk (or discussion) page before you make them, you should reach consensus faster and happier.
5. Respect copyright. Wikipedia uses the GNU Free Documentation License. Everything you contribute must be compatible with that license.
6. Decent edit summaries are universally appreciated. Other editors need to understand your process, and it also helps yourself understand what you did after a long leave of absence from an article. Please state what you changed and why. If the explanation is too long, elucidate on the discussion page. It is a fundamental principle of Wikipedia that anyone may edit articles without registering so there are lots of changes to watch; edit summaries simplify this.
7. Don't revert good faith edits. Reverting is a little too powerful sometimes. Don't succumb to the temptation, unless you're reverting very obvious vandalism (like "LALALALAL(*) or someone changing "6+5*2=16" to "6+5*2=17"). If you really can't stand something, revert once, with an edit summary something like "(rv) I disagree strongly, I'll explain why in talk." and immediately take it to talk.
8. No Personal Attacks. Don't write that user such and so is an idiot, or insult him/her (even if (s)he is an idiot). Instead, explain what they did wrong, why it is wrong, and how to fix it. If possible, fix it yourself (but see above).
9. Be graceful: Be liberal in what you accept, Be conservative in what you do. Try to accomodate other people's quirks as best you can, but try to be as polite, solid and straightforward as possible yourself.
10. Signing. Sign on talk (using ~~~~ which gets replaced by your username when you hit submit) , but don't sign mainspace articles.
11. Foundation Issues: There are only 5 actual rules on Wikipedia: NPOV, a free license, the wiki process, the ability of anyone to edit, and the ultimate authority of Jimbo and the board on process matters. If you disagree strongly with them, you may to consider whether Wikipedia is the right place for you at all. While anything can theoretically be changed on a wiki, the community up to this point has been built on these principles and is highly unlikely to move away from them in the future. A lot of thought has been put into them and they've worked for us so far; do give them a fair shake before attempting to radically change them or leaving the project.