I want three things from my tank (IDC what class at all):
1 - focus on defense more than HP... I've had a couple higher HP tanks which seemed to mean they had lowered defense and took more damage, required more healing, and I had more aggro.
2 - Know what your limitations: if you pull to groups of mobs and I can keep up, go right ahead, if you can't pull one group of mobs without CC then be careful!
3 - Pay attention to me (okay this one goes both ways) but I want the tank to be aware of whether I'm keeping up easily, or hes almost dieing every pull, or he's losing threat
