Tactful ways to inform people that they're playing badly?

[SerLuke]

Tactful way to inform someone they are playing badly:
-Leave

It may mean you have to requeue in 30m but would you rather sit in a dungeon with a bad player? or would you rather do something for 30m and then probably get a better group next time?

I dont even bother anymore. I just leave them. (which gets the point across even more if your a tank or healer)

You would rather take a 30 minute penality over a 20 minute dungeon run?