Hunters using mana and swapping between Aspect of the Hawk and Aspect of the Viper throughout a fight. Also needing to feed your pet to keep it happy for the +125% damage buff and purchasing ammo.
Warlocks collecting soulstones before raids.
And... the big one for me --- All 3 DK specs being balanced around both tanking and dpsing. Dual-wield Frost Tank? Go for it. 2H Unholy Tank? Sure! --- It's a lot better now than it was then. But it was very interesting to have all those options. (Even if one was still the "best".)