SynCaine of Hardcore Casual speaks to another writer's statement that "bad players are bad people."
This made me think about what makes for a Good Player. This is within the context of running dungeons, something I've been doing in World of Warcraft quite a bit lately, and though I finally recently earned the title "The Patient" I'd be surprised if most players have been in more PUGs than I have over the course of years.
I've played every single role, some better than others. That has helped me immensely while running dungeons, because I know what each class can do in their role, and I can get a sense if a player needs extra help from my characters, and I know where the points are in most dungeons where things can go south, and be hyper vigilant.
Here's what makes a Good Player
Do the best you can in your current role. Work on a good rotation of skills, know your stat and armor requirements, carry potions to get you through bad spots.
Provide support for the rest of the group.
Be attentive to everything going on, not just what you need to do, but be aware of where the other players are and what they're doing to help or harm the group.
If you provide guidance to another player, do it in such a way that it is helpful to them, and it sticks.
Learn the dungeon/encounter.
As DPS, protect the Healer, assist the Tank.
Play all the roles so you know what is expected and possible.