In normal combat, my mage is almost casting spells 100% of the time, once I got into my teens. You want to pick up the frost mage 50% regen ability along with the fade walk thing. Then with fire, when you crit it lets your next spell be cast without a cooldown and I have 46% chance to crit. That means I get a crit almost every immolate and fire mine I cast. Then, because of mana difficulties, Rift mage has an ability that gives you mana based on 10% of the damage you do against weakend enemy (all Rift mage abilities weaken), so that always caps off my mana when casting stonefist. As a result of the build, I am casting almost 100% of the time. The only time I'm not is against Dragons, as I'm bouncing between all my characters trying to position them.
Edit: I was just playing, and noticing that my mage casts about 90% of the time. I'll usually get in about 5-8 spells before I have to slow down most time. My typical start is Pull of the Abyss, Fire Mine, wait 1 second, Stonefist, then often, I get in a crit cycle with stonefist or fire mine, and cast about 3 more spells before I slow down. Of course, most the time everything is dead by then, but different resists sometimes play a factor.
Now you did mention something I do find annoying, and that is games which make all 3 archtypes fight the same. Where mages, rogues and warriors all feel like the same character. I think the absolute worst example I've seen of that was Asheron's Call 2. I can see how it applies a bit here, but they do all seem to fight different from my experience.