Fake/incompetent/under geared/low cp tanks, healer, and/or dps.
If I come to the conclussion that I have to spend more than 30 minutes in a dungeon, or that I might have to teach the group the mechanics. I leave, I am not in the business of free carry runs.
Depends. If they are nice and try their best, I stick with them. If we don't progress, they usually realize they're not ready for the content and give up long before I do. If they have a bad attitude, are horrible at their role and refuse to leave, I will vote to kick them.