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.
This works fine for books but I hate it for TTRPGs. Anytime an RPG wants to punish you for using the cool powers it has provided I roll my eyes. There are a few settings where this works well, Unknown Armies comes to mind, but in general if the RPG says "Here are some cool powers but be careful not to use them!" I say OK and then close the book and reach for a different RPG.
For clarity, the game I had in mind there is Magicka. You're not directly punished just for using magic, but all spells carry risk through something like area effects, ricochet, trapping yourself behind obstacles you place, mis-typing input combos, idiosyncratic targeting mechanisms, trying to cast electric/fire magic while you're soaked with water/oil, etc.
Trying to hit the lich with a thunderbolt? Congratulations, you just exploded your party's archer because you forgot they were at the highest elevation of any character in range.
Also for clarity, I don't think this system is appropriate for a ttrpg. It really needs time constraints to work properly, or harder-to-avoid risks that start to verge into "direct punishment" territory, which itself has a high risk of just not being fun. I think the latter option is generally best saved for the most devastating (or just eldritch/necromantic) spells. You can summon an army of beholders, but the material component is your character's own eyes.
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 Lancerdeleted by creator