Wandering from blog to blog a few days ago, I stumbled upon a free download of a fantasy RPG called Errant. Reading through it, the feel I got was very similar to that of D&D, with some notable differences. One of the interesting mechanics was using hit points as spell points, with casting cost of 1 hp per spell level.
This models quite well the fictional portrayal of spell casting as a physically exhausting endeavor. There are some aspects of it that I really like, and some that I'm not so sure about with regards to porting it over to a more traditional D&D framework.
It maintains the resource management aspect of spell casting, without the rigidity of a table of spells per level per day.
The bookkeeping is minimal. In fact, it eliminates a whole category of bookkeeping in the game. You already have to keep track of hit points anyway, so it adds virtually nothing to overall complexity.
It would technically give low level magic-users the potential for casting more spells than the traditional Vancian tables allow, but at the same time it provides a very strong incentive to stay well below that limit most of the time.
It provides the potential for epic sacrifices and desperate ploys, as a mage burns his last hit points on a spell to save the day.
It strengthens the incentive for magic-users to stay out of the front lines of combat.
It provides for interesting tactical choices for a worn-down party: Do the healers refresh the fighters, or recharge the mages?
You can adjust the lethality of overreaching one's capacity according to your own preference. Dropping to zero hp from spell casting might result in death, unconsciousness, or a saving throw with success indicating unconsciousness and failure death.
It obviously won't work for clerics, or for any class that can cast healing spells. Even barring a caster from casting such spells on himself isn't an effective control. Put two healers in a party, and you've got a perpetual casting machine.
Casters with high Constitution scores are strongly favored. In fact, Constitution becomes more important to a magic-user than Intelligence. Errant keeps Intelligence relevant by requiring an ability check to successfully cast the spell in the first place, but I think that overcompensates and ends up overemphasizing a high Intelligence score. Perhaps using Intelligence as a chance to learn a spell, and/or a cap on number of spells known, would strike a better balance. A high-Con character would have an early advantage, but the high-Int character becomes far more versatile in the long run. Of course, it's possible to have a magic-user who's both, but 3d6 in order makes that unlikely.
Classic D&D elves are a problem, because they have more hp than a magic-user, and thus a greater pool of spell points. This might be mitigated by charging an extra point per spell level, or even doubling the cost, for elves (and other multi-class magic-users, in an AD&D-style game.)
Hit points vs. Vancian slots
Here's a side-by-side comparison showing the number of spell levels available to Vancian magic-users of selected levels vs. the average number of hit points. (Mentzer rule set)
Level Vancian spell levels total Average hit points (rounded up)
1 1 3
2 2 5
3 4 8
4 6 10
5 9 13
7 17 18
8 23 20
9 31 23
14 67 28
21 141 35
36 405 50
Hmmm...that's great for low-level magic-users, but it quickly becomes a substantial handicap at higher levels. It turns out that magic-users gain spell slots a lot faster than they gain hit points, and above name level, it's one paltry hit point (i.e. one measly spell level in an hp-as-sp system) per level. A 36th level magic-user in the original rules can cast nine spells of each level per day; with the hit point system, he's running on fumes after casting only one of each level. Granted, he's gaining something in versatility, being able to choose whatever spell fits his needs a the moment, but at what a price!
Maybe it's not as bad as it seems. In the spell slot system, a caster has to memorize spells in advance, and either correctly anticipate which ones he'll need, or choose a broad selection, with a lot of them likely going unused on any given adventuring day. In the hp system, if he needs five Knock spells, he has access to them, and doesn't have to anticipate the need for them or load up his brain with a lot of superfluous stuff. In other words, most of the time, just because a magic-user can memorize a lot more spell levels than a hp-fueled magic-user can cast, it doesn't follow that the Vancian wizard is necessarily going to cast a lot more of them. Combined with the fact that healing magic actually restores the hp wizard's reservoir of spell power, it might actually be pretty close to a wash.
If all else fails, perhaps (if I may steal a mechanic from another source) magic-users should be allowed a saving throw vs. spells to avoid the hp drain, with a penalty equal to the level of the spell being cast. Low level casters would still lose hp on most casts, while high level ones would feel the drain less often.
Even if I decide that extra roll is needed to keep things close to the original balance of power, it still might be worth it not to have to do all the accounting for spell slots and memorization. Painstaking selection of an extensive list of spells every morning of game time (and for every NPC magic-user,) or roll a single die once and subtract hp or not each time a spell is cast...that sounds like a fair trade.
Despite my misgivings about the possibly excessive gimping of high level magic-users, and my fondness for my recently posited spell component-based system, I think this just might be the way to go for my group, which includes my young nieces whom I very much doubt have the patience for a lot of bookkeeping. Now I just need to find a similarly elegant mechanic for cleric spells, to avoid the healing conundrum (and in the process, maybe even provide a really substantive difference from arcane spell casting other than just different spell lists.)