On a more serious note:
1. Ability to listen and build on other players' input, instead of blocking them.
2. Quick thinking; being able to react to what's happening in game without getting passive or going on autopilot (which often means "mindless violence").
3. Courage to do what's interesting, not what is tried and safe.
4. Proactivity; setting own goals and pursuing them.
5. Flexibility; being able to play various types of characters, in various settings and playstyle. Also, being able to achieve compromise with other players without violating own character's concept.
6. Brevity; being able to express the character, their intentions and actions in a few words instead of long monologues.