I am against reagents/material components for spells because it's basically mana in a more annoying, inconvenient form that confuses people and clutters up the inventory. At that point it's easier to just bring Mana back, and add a 10-turn cooldown to drinking mana potions.
I do like the idea of spells that take one or more turns to prepare before they're actually cast, and I suspect that Larian is also considering that.
I think that longer cooldowns for more powerful spells is another good answer. Forktong's objection to that idea is that people could just sit around and wait for them to recharge. I think that if they do so, that's their prerogative, but some people might not want to wait around for 5 real-time minutes (or more) doing nothing.
As I already wrote, cooldodwns simply aren't effective outside of combat, especially in a game with a low encounter rate like Original Sin. Dungeons aside, you have plenty of things to do between combats,so longer cooldowns wouldn't effect your habit about ability spam. Furthermore Forktong is right: an easy exploitable system is bad design no mater what. "It's their prerogative" is not an excuse.
As for the mana/reagent/whatever management: this is not the most elegant solution to the problem, but it IS A solution, as many good games already proved.
I prefer trade-offs, though, because: a) they are interesting mechanics per sé b) they balance combat without adding additional systems to the game. Different casting time for different spell level is an interesting mechanic, really hard to balance though.
