I have been playing WoW for quite a while now (nearly two years) and I have had a chance to play around with Warhammer Online. What always draws me is, of course, the various caster classes.
I did mention that I played Warhammer Online for a month (free trial) and while I chose not to continue playing (the game doesn't fell as polished as WoW at the moment, and it feels a bit less fun), trying out the various caster classes was fun.
From the Dark Elf Sorcerer to the Orc's Shaman, it was an interesting experience. What was odd in Warhammer Online, was the universal Action Point bar for each class, so everybody relied on it to some extent. What was nice though, was that each class Archpretype had a unique mechanic, so that made casting interesting and varied.
I have had a lot more experience playing the caster classes in WoW though- In fact, I have a level 30 Warlock, a level 22 balance Druid, a level 7 Priest ( I have had more but they ended up deleted) and a level 17 Elemental Shammy. As you can see, all the hybrid classes are specced for casting, not melee, as I like nuking things more than beating things (at least as a hybrid). I have enjoyed playing all of them (except the priest), and I have plans to continue with my warlock at some point. What I can say is that while each of these classes use the mana bar, they all get varied ways to nuke things to death and each one feels unique to me.
Of course, I always end up back at my mage, because unlike the others, I get three talent trees to do one thing- blow things up!
Image from- http://dragonsdenls.com/images/Sorcerer.bmp
-Eric