To answer the OP's question, I would say a good player is aware of their surroundings, knows at least roughly how to do the dungeon or raid they are about to do, asks questions if they are unsure on how to proceed the next section/boss, has an understanding how to get the most out of their char (rotations and cool down priority), has the correct type of gems in their equipment and finally is will to listen to any advice given to them by other players if they are having a problem beating something in the game and a simple alteration could fix it (this is a bit tricky to know if you're being told something truthful.

I always try be polite to my fellow party members and will always warn the group if I am rusty with tactics on a dungeon if I've not done it for a long time. (I tend to take a couple months break at a time, so I do need the odd refresh on some of the least common dungeons when doing a rootlet via the party finder)