As a healer as well, pretty much this.
Tanking tips:
Say at the start of a dungeon, mainly if it's a level 30 or under dungeon to attack the marked targets in order.
Ask them to only hit the target that you are and hope that they listen. I have a 1, 2 and 3 mark on my bar and always mark each target before we pull.
If it comes to it, let them know to let you pull. Luckily I have yet to run into a ARC or any ARC at all, really, that pulls.(Does no one play this class?)
If you are a GLD/PLD and expended all your mana, ask them to wait and let you refill it if you didn't manage to get it back up.
*If you see after the first pull that someone doesn't know their job, politely ask the group to stop pulling and explain their class role nicely.
But most importantly, if one person is messing up, please try and power through it.
If you leave or let them die, you are hurting yourself and 2 other people as well, not just him.