One of the things that's always bugged me about D&D's "Vancian" magic system was that, while the game mechanics are the same for both magic-user and cleric spells, the explanation for why this is so is fuzzy. If spells take up mental "space" in the process of memorizing their arcane formulae, then does that mean a cleric is simply a magic-user by another name, that is, another flavor of wizard? Otherwise, it would seem to me that clerics, who simply pray to their deity to grant them their spells each day, ought either to use a different mechanical system or a different explanation of how they obtain their spells.
For Pulp Fantasy D&D, I opted to keep the standard Vancian system for clerics but to embrace the notion that they were simply magic-users by another name. Whereas wizards use their Intelligence to command the powers of the cosmos, clerics look to their Wisdom to find ways to bring themselves and their companions into accord with it. Consequently, a cleric's spell list is almost entirely protective/creative in nature rather than destructive, as a wizard's is. What about reversible spells, you may ask? A good question. In OD&D, clerics cannot cast reversed versions of their spells, but anti-clerics can. "Anti-clerics" are Chaotic clerics who can only cast reversed spells. That is, they can only cause light wounds rather than cure them. They can likewise never even raise dead, which has interesting implications for the campaign world.
I like this idea in principle, but it doesn't quite jibe with the way I want to explain why wizards and clerics use the same game mechanics for spells but get very different types of spells. So what I'm doing is this: regardless of alignment, the cleric's spell list does not include reversible spells. The path of the cleric is one of healing and defense. Even the bad guys need healers and the cosmos doesn't care whom you're healing or why, only that you are. However, reversed spells are still possible. A cleric of any alignment can still choose to reverse one of his spells, but doing so opens his soul to darkness, since he's betraying the very intent of the path he has chosen, in effect trying to trick the cosmos through his superior insights into it.
Each time a cleric casts a reversed spell he must immediately make a saving throw against magic. If he fails, he loses a point of both Wisdom and Constitution. This loss is either temporary or permanent depending on what the cleric chooses to do between now and the next time he attempts to memorize his spells. If he wishes to overcome the ability score loss, he can atone by undertaking some sort of perpetual penance that consists either of an action -- never again to eat meat, never to wear purple garments, always give money to beggars, flagellate himself daily --
or a religious devotion -- carrying the relic of St. So-and-So at all times, always greeting others in the name of his god, blessing any places he enters, etc. These penances can never be abandoned, as they are the only thing that keep the wrath of the cosmos at bay. Should a cleric be unable or unwilling to perform one, he will then lose the Wisdom and Constitution he lost before, only this time there is no restoring it. A penance must be performed for each and every time the cleric casts a reversed spell and fails his saving throw, meaning that clerics who make a habit of casting reversed spells will soon either have very low Wisdom and Constitution scores or be a bundle of taboos and eccentric pious behavior. Any cleric whose Wisdom or Constitution score drops to zero dies and rises as an undead being whose Hit Dice is closest to their level at the time of death.
Because my pulp fantasy clerics are closer to wizard's than to the traditional D&D Knight Hospitaller, I've decided to limit their choices to those of the wizard. In almost every other respect, though, they're identical to the D&D cleric -- spell progression, weapon restrictions, hit dice, the ability to turn undead (since the undead represent the ultimate example of beings "out of sync" with the cosmos) -- but I've reworked their experience table thusly, to accommodate the fact that they have fewer armor choices:
1 - 0
2 - 1400
3 - 2800
4 - 5600
5 - 11,200
6 - 25,000
7 - 50,000
8 - 100,000
9 - 200,000
+100,000 XP per level after 9th