1. R-e-s-p-e-c-t. If there's one thing a bad player does often, it's disrespect other players. WoW is a game where you're meant to play together well, and that's not going to happen when you ask a warlock for a portal, and then get angry when he can't do it (see #3 about that). Yelling at people (a.k.a. using all caps when you speak), assuming help, ninja-ing loot or resource nodes-- these are all signs of disrespect. Blizzard has built all sorts of benefits into helping people in this game, so chances are if someone isn't doing something for you, or isn't helping in some way, it's because you haven't given them the respect you should.
2. Help yourself first. If no one can help you, don't just get angry about it-- help yourself. If you can't find out what do to for a quest, look on Thottbot. If no one wants to run an instance, find something else to do, watch the LFG channel, and before you know it, somebody will be looking for exactly the thing you wanted to do. If you really, really need 1g, just step out of town and kill a few things until you get it-- I'm not kidding, money is just lying around on dead guys in Azeroth. WoW is a great game in that you can't lose-- no matter how broke or low level or low geared your character is, there's always a way for you to get better, even without anyone's help.
3. Know the game. Everyone says this, but everyone says it because it's true. There are countless resources out there to help you play your class-- read at least a few of them. If someone does something you've never seen before, nicely ask them about it, and they'll probably talk your ear off about all the stuff they can do. And if you're in an instance and you're about to fight a boss you've never fought, let your group know so they can fill you in on what to do during the fight.
4. No begging. This falls right in with #1, but it's worth repeating. In life or in Azeroth, don't beg. There are always better ways to get what you want (see #2).
5. Relax and keep an open mind. One of the biggest complaints this week was about people who were angry because one of the other players wasn't doing something they thought was right. Warriors dual wielding with daggers?!? Druids tanking instances?! Priests pulling!? And yet all of these things aren't just possible-- sometimes they're the best way to do what you need to do. Don't get angry just because someone isn't sticking to the cookie cutter classes-- if everyone lives, it works. And even if everyone doesn't live, don't freak. Wiping, leaving a group, even a huge repair bill doesn't equal the end of the world (that's for when the Burning Crusade shows up).
There we go, a constructive end to a week of complaining. And if someone doesn't follow one of these rules, you can send them right back here to find out where they went wrong.