I don't think the question is specific enough. There's a difference between a "bad player" and a "player you don't want to play with".

I've been with players who could heal and DPS with the best of 'em, but their attitudes were so terrible that you just didn't want to be around them, eventually. Great player, but not great to play with. On the other hand, I've run with people who didn't know how to silence or stun and who turned mobs towards the party rather than away from it, but whose /smile could light up the whole dungeon. They weren't great players, but they were great to play with. And if they were willing to learn, well, I was more likely to call them a good player in the long run.