From the perspective of a DPS, I really don't like it when the tank standing in the center with mobs in front and behind them. They get spaced to far apart for my Aoe's to hit them all at once, and also too far apart for me to switch targets and interrupt a mobs attack with my stun. Also I'm not sure if block/parry/dodge trigger as often when things are poking you in the back, so it could be making things harder on the healers as well.

Also I don't like it when a tank becomes too tunnel visioned and decides to park a mob in front of some obstacle. Sometimes it s a wall that prevents me from getting behind an enemy. Other times its a pool of fire or something on the ground that I can't stand in without taking damage.