Well Arcane mages had skills that used increasing amounts of mana, and then skills which gave them mana back... So that comparison still sort of sticks, for me :-P It's basically an immobile glass cannon with huge mana-balancing responsibilities. Outside of that, not that complicated to play.

I don't actually think there is anything that plays like a Demo Warlock in this game, right now. Plus, pure DPS classes in WoW have different play styles, depending n spec. In FFXIV, you just have to play the hand you're dealt based on the job you choose to play. There is not much inter-job variety in gameplay.

Can always just avoid Demo in WoW by going Affliction or Destruction. Black Mages in FFXIV have to basically play another class (i.e. job) altogether. You still have to level it, so the fact that you do it on the same class doesn't mean much... You just get the benefit of maybe using gear you have already farmed if you were playing a class that shared gear with BLM.