What a loaded question. A good player is someone you wanna play with and that type of player varies from person to person. But the consensus is at least knows how to play, behaves well, and is actively participating. Personally I wouldn't put skill level as the top quality of a good player, but being considerate of others, you know, the attitude that actually makes playing a game with others playable.
Also, the funniest moments I had in duties were when players made mistakes. You'd hardly see that happen with very experienced players but fortunately they pull the weight of struggling party members. All these types of players make the game interesting. Just be a good sport and we're all good.
