Mana

The second computer my family ever bought, the one on which i started really getting into games, was a Pentium with a dizzyingly fast 166 MHz processor and full VGA graphics capability. It came with a few pieces of software to sweeten the deal, among them two games: PGA Something Tour and Magic Carpet.
This post is not about the golf game.

Magic Carpet did not hold my attention much. It was a simplistic twitch-game FPS and while the novelty of flying around shooting fireballs took quite a bit longer to wear off than that of... putting... it did little to keep me interested in the way that Mechwarrior 2: Mercenaries, C&C: Red Alert, Sim City 2000 or Starcraft did.

However, Magic Carpet did keep to one important concept which fell by the wayside in later years' FPS and RPGs. Spells (fireballs) were cast using mana, but mana was a finite resource, not an infinitely regenerating gigantic pool from which you could shoot fireworks almost indefinitely. It was a physical (spiritual?) resource actively gathered as you played, jealously hoarded and fought over, a la Magic:The Gathering. It was not a meaningless freebie implemented just for show.
Even the game's backstory involved mana becoming the 'spice melange' of the world. He who controls the stuff you use to shoot fireballs controls the universe.

That was the style at the time. Other series like the original Diablo or TES 3: Morrowind also used a finite, actively replenished mana pool, while Diablo 2 and TES 4: Oblivion did not. The audience gradually decided that keeping track of mana was just a lot to think about and we all know thinking causes school shootings. The death of the mana-as-resource concept was merely a minor side-effect of the overall simplification of computer games to appeal to the moronic masses. Let's revive it.
Come on, that spell can't have more than a 47-mana cost. 

However, in order to bring back a meaningful use of mana, one would have to apply the same necromantic ability to the entire over-arching concept of resource management, from mining nodes to depletable quivers of arrows and guns with bulky packs of ammo which need to be carried around. Resource management requires foresight and planning and that requires thought and thought, well...

causes fireballs.