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.
Personally I quite like them, but I feel like they should be long-term effects rather than just "you rolled a 1 so you miscast and hurt yourself." The long-term effects can add a lot to RP potential for characters that I think makes it interesting.
Like, as an example, say magic ages people faster, so every time you level up you also age a few years. Or, to take from Dark Sun, magic gradually transforms you into various magic creatures, taking away the caster's humanity. Both could lead to some interesting RP that isn't super common in most games. (Or, well, the former isn't, the human/inhuman thing comes up in WoD stuff all the time)
Obviously in such a case you would require a different system to limit magic use for balance though, so we return to the question of slots or mana
or scene abilities like in 4e or Lancer