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.
Agreed. Spell slots are a weird holdover from early D&D, and they have a lot of negative knock-on effects.
For instance, a system where characters have abilities (like magic spells or superman martial feats) that can be powered by a resource pool (like mana or willpower or whatever) would be a lot easier to fix the martial/caster asymmetry problems that continue to plague D&D.
On the other hand, there's something interesting that some OSR games like Knave and Cairn do where all spells are magic items (like a magic tome) that uses up an inventory slot. Rather than having magic as a class ability, plays have to chose to use their inventory on either weapons/armor, magic spells, or other useful tools/gadgets that leads to character specialization.