If I'm Healing:
Tank : It's my job to keep you standing. As long as you only pull one group of mobs at a time, you should never fall down. If you do, feel free to give me a hard time.
DPS : It's your job to do damage to the mobs without pulling aggro. I will try to keep you up, but you are not my healing priority. If you pull aggro off the tank, you are wasting my mana, and I may decide it's cheaper to rez you.
If I'm Tanking:
Healer : It's my job to keep the mobs off you. If a mob kills you, it's my fault. Feel free to give me a hard time about it.
DPS : It's your job to do damage to the mobs without pulling aggro. I will mark a target with a skull in each pull, and rotate to a new target when that one dies. If you are damaging targets that haven't been marked with a skull, and you draw aggro from them, remember the aphorism "you spank it, you tank it". AoE at your own peril unless I've said otherwise.
If you pull a mob with a taunt or direct damage before I pull, I am more than happy to assume you want a shot at tanking, and will gladly dump my aggro and shift to DPS form.