To me what makes a good player is someone who, despite lacking some skill in aspect of the game tries to make up for it by genuinely trying to play as a teamate or pick up things about the game quickly to adapt to his team's needs.
For example: (since i've been playing Left 4 Dead 2 alot recently) a player who is visibly noticable for having a sub-pair aiming but still sticks with the team, points out items and threats as well as providing support when a team member is downed or in need of a heal.
I'd take that guy anyday over some guy who can nail a hunter mid-air across the map blindfolded with a pension to rush and generally dis-reguard his team as a whole.
Edit: Modesty is nice as well, i'm often taken aback when someone says "Good game" after a match and try to return the goodwill