Must have: Pride

Just like in real life, people should have pride in what they do. When we do something, we should focus on doing it right, and not do it just because we have to. Doing your job without even caring for it just shows how much of a "slacker" you are. If you don't care about doing your best, and everything you only care about is your pay check, you obviously think the same way when you're playing World of Warcraft.

If you're a raider, you must do your best, both before, during, and after raids.

• Before the raids, you should make sure your gear is enchanted with the best available enchants for your role, you should be properly gemmed, you should have potions, flasks, elixirs, buff-food, and every sort of consumables to maximise your presence there. Those few extra stats like AP, SP, MP5 (etc.) you will get may not sound like much, but if everyone in the raid has those extra stats, you'll most likely progress easier.
  
• During raids, you should pay attention during the encounters, you should know your spell rotations (which must be studied deeply before the raids, obviously), and you mustn't stand in the fire. Knowing tactics is a must.
  
• After raids, you should enchant/gem any new gear you got, and also reflect on what happened during the raid. Did you do the best you could? Did others do the best they could? Any raid tracking tools are helpful for this. Numbers actually say a lot about someone.
  

I'm proud to be a good healer. I'm not cocky; I know my skills, and care a lot about my performances. When I compare myself to other healers, I often am on top of the effective heals (I never out heal a Priest, though), and got one of the lowest overhealing. When compared to other shamans, my Earth Shield often healed a lot more than others' (because I focus on ALWAYS keeping it up), I had more Critical heals (which also cause Ancestral Healing), and my Riptide ticked more often and for higher amounts (Riptide also causes Tidal Waves, which enhances both my HW and LHW. I choose which one to use according to the situation, of course). No special skills required really. To be good at it, I'm properly enchanted, gemmed and flasked, everytime. I'm always looking to learn more about my class, and there's always room for improving, either rotation or stats wise.

It's always about motivation; you don't need to be one of those obsessed gamers (who play 24/7) in order to show your skills as a player in WoW. However, you need to keep in mind that you can't achieve success on your own, so help others helping you (considering you're in a guild with good players), by doing it right. And does it feel good when you put out a hell of a performance!

Raiding without doing your best is irrational, because that means you're leeching skilled people, and not allowing them to progress, just because you expect easy purples. Why would you play this game after all then, if all you want is gear to show off to people you don't even know and don't care about?