Over at Blog Azeroth, the current Shared Topic of the month was started by Jaramon from DeathCoil.org
In his post, he asks himself and the readers: how do you define a good player? This calls for a very subjective answer.
One can interpret 'good' as 'good' in a moral sense, in which case a good player is, in my opinion, one that is not loot-greedy when raiding, not spiteful when killing or being killed in PvP, and not attention-seeking in RP (and preferably all of these together). A good player in the moral sense is someone you'd want to hang out with, someone you'd want in your guild. A social, friendly and helpful player.
Sadly, one can see 'good' as 'good for', a utility. For example, recruiting someone who is clearly very annoying, but because you need his specific role, you recruit him, because he's 'good for tanking' or 'a useful arena healer'. You don't really like the guy, but he's useful, so you call him a 'good' player. This isn't really the kind of relationship you want to have with other players, you just need to make friends with enjoyable people that also happen to be of the class/role you want. After all, what's more fun than just rounding up a bunch of friends and having a great time?
The final interpretation of 'good' is 'good at'. This involves skill and expertise (in [role]playing your character). This is very subjective of course, but in my opinion a 'good' player is one that is aware of what is happening around him (fire/goo on the ground, an enemy player bandaging under the bridge, a healer about to get squished), that knows what he's doing, and can anticipate things, like refreshing Slice and Dice right before you run out of an AoE of which you know it will come, or throwing on some HoTs right before a raid-wide silence.
This is also the most controversial definition of 'good'; because while you may think you are good at something, others may not agree (proof of this can be found daily on the official WoW forums). While a group of semi-casual players clearing 10-man Naxx after a few times may think they are good, a semi-hardcore group clearing 25-man Sartharion with 3 drakes may argue they still have a lot to learn.
As a closing statement, I would like to add that I found a lot of this reflects my guilds philosophy:
[...]but because in the end, this game is about people and about having fun. Our main policy has always been 'People over classes/players'. Playing skill can be learned, and the character you play as your main can be changed, but being a good person - that's not something we can teach you, but rather an important part of finding a home with the Athanatoi. We expect maturity and good behavior from our members[...]
Speaking of which .. happy second anniversary, guys!
Aeltyr slips his letter in the mail, steals a small slice of birthday cake and quietly sneaks away.