In the time of random dungeon tools, dungeon levelling and pug-able raids it is easy to lose faith in humanity. If you read this guide and pay attention to what it says I am sure that your PvE life will be easier in many ways. It is mainly aimed towards end-game raiding but most of it also applies to low level dungeons. Feel free to spread this guide as long as you tell that I (Menyet, Defias Brotherhood) am the author.

I will not write much about gear, specc and stat preferences as I assume the class forums cover this up well enough.

General Behavior

General politeness is a good thing. Greet people when you join the group. Say Gratz when they level up. Talk to them! It makes PvE so much more fun. And never ignore the chat, both important and hilarious things are likely to be said there. PvE is team work and should be treated as such!

Do not expect everyone to be seasoned players. There are loads of new players that have never been to dungeons or raids before. Do not nab out on them if they do mistakes but politely tell them what they can do better. If they still don't do it, tell them again. If they still don't do it you can freely assume that they don't care or are jerks. Although, this is a 12 years rated game and you will encounter children playing. Keep this in mind as well as the fact that many 12 year old children haven't had english classes at school for more than a couple of years. Be fair.

Need before Greed. Need means that the item is an upgrade to what you are currently wearing for your main specc. There is no such thing as need for gold/alts/enchanting mats. Off-specc (for example DPS if you are a healer or tank) needing is also frowned upon. If you would like something for your off-spec you should wait until everyone else rolled and, unless someone else pressed need, ask if they are fine with you needing for your offspec. You should also not need for armor that is below you, for example paladins rolling on mail items (after level 40 when they get to wear plate) or shamans rolling for cloth. If it is an upgrade, do as above with off-spec.

Decide what role you want to play and keep it that way! Healers heal, tanks tank and DPS DPS. It is as simple as that. At early levels speccs are not as important and a shadow priest can easily heal, although probably mana starved. At late levels the specc gets vital, though, and it is not only rude but also very stupid to sign up as a tank when you are a retribution paladin, or DPS as a restoration druid. Dual-speccs are available at level 40 and are a nifty thing - as long as you remember to also swap your gear!

Come prepared!! This is so important it can't be stressed enough. Always bring potions, flasks and food/drink enough for the raid. Come repaired and read up on boss tactics before! If you for some reason are not entirely sure of what to do - ask questions until you are! Silly questions are way less embaressing than wiping the entire raid because you didn't have a clue.

Fire is bad for you. If your character is standing in a fire, if the earth moves beneath you in curious ways or if some huge green cloud is heading your way I think you can safely assume that it's not going to be a good thing, right? So move and save the raid leader from getting a sore voice. Just paying attention to what happens on the screen is key to good players.

Do not be afraid of vent. Vent is there to make communication easier and faster. It is a light-weight and safe program and it will not only make raids easier but also way funnier. If you don't have a microphone or don't dare to speak you can at least listen to the raid leader. Tanks should always have a microphone, though, as they will need to announce things like debuffs, taunts or similar. Headphones are better than speakers, especially if you have a microphone as the sounds played in your speakers would duplicate painfully in your mic. Use push-to-talk buttons and no one has to hear you burp and fart or yell at your mum.

Learn about other classes and speccs. Not saying that you should know everything about every 30 speccs as that is hard even for the Blizzard employees. But you should have a general idea of what they do and how they work. For example how shadow priests mainly use DoT's for damage, feral tank druids can have problems AoE-tanking or holy paladins are more suited for main tank healing than resto shamans. Knowing how others will likely behave will make it easier for you to understand what happens and why or, most importantly, what you can do to save the group if something goes terribly wrong.

Gear has very little to do with a person's abilities. Of course hunters shouldn't wear robes and healers shouldn't wear strength gems, aswell as you wouldn't bring a clearly undergeared person to a raid. That is also were gear stops being an indication of a good player. Just because someone has the best gear available it doesn't mean that they are good raiders, can take instructions, keep out of fire or pull their weight. These days they might just have farmed heroics or been lucky in VoA. Personally I would rather bring 24 polite people who are willing to do what it takes than 24 nabs that don't give a damn.

Show up in PvE specc and PvE gear. I bet you pwn in PvP, but PvE is a completely different thing. PvP specc and gear will likely cause you to oom faster, do less damage and generally be worse than others. Don't do that to yourself.
Roleplaying is like sex:
If done with the right person it is fabulous and I never want it to stop - with the wrong person I just want him to be done and log out.