Playing Pool Of Radiance got me learning the ins and outs of 2nd edition AD&D and I came to the realization today that I prefer games with a magic points or mana system. Probably because I grew up with JRPGs which exclusively deal with MP over spell slots. Don't get me wrong, D&D is great, but it's such a pain when you get into battle and you realize you forgot to memorize Detect Magic and now you have a bunch of potential good loot.
Your suggestion remind me of the "reverse mana" system from the various Ogre and Final Fantasy Tactics games - start with 0 mana and gain some each turn, so you can keep casting simpler magics, or save up for a more powerful spell.