Continuing with the basic concept of What makes a player great this skill is about what makes a player a Team player. Adventuring groups have strengths because they aren’t cookie cutter copies of each other. Sometimes those differences drive Players and characters apart. Some mechanics in some rules even help form divisions later in a campaign.
Naturally, with these and many more social and behavioral temptations in and out of game only disciplined players can find not only resistance to self interest but willing to take an active interest in helping the other players achieve their goals. Even at the delay or cost of their own immediate and sometimes long term goals. Of course if everyone shows the same skills then negotiation over when personal goals get solved as opposed to if the goals will ever be acted upon.
When playing or judging others as players the following skill should be considered lest a team founder upon the rocky shoals of a trust where there should be smooth sailing:
Teamwork (group/party support and cooperation) A top notch gamer can provide support no matter the position to both the GM and their fellow players. Even positions considered typically a non-support position (thieves and fighters as a potential example) by letting others shine or letting their preferred choice go when it is better to act cohesively than delay of game to argue for the personally preferred direction.
Related to this question of how much teamwork a player shows is how much is a character willing to go through? This is a tough question and to a certain extent reflects upon the player as well. In part is the character so closely tied with the player that on a scale from ‘is no tragedy too great?’ to ‘nothing bad must befall the character!’. Knowing where each player (with a particular character) falls upon that scale is a good, and where the player is unchallenged, comfortable, uneasy and unhappy is also a point of consideration for a wise Gm and group.
Which brings us to the picture topping this post. Iron Matilda was a character of mine who fought in the Temple of Elemental Evil campaign for quite a while. At one point she snapped the Wand of a-Wonder taking out 10 Ettins in so doing and suffering just about every imaginable indignity at once. Then things took for a turn to the worse… leaving me the time to draw this and another piece about my characters sufferings.