I've frequently heard a sentiment that Pathfinder, the 3.5e update created by Paizo Publishing, doesn't accomplish what it sets out to do, i.e., fix 3.5e. It fixes polymorph, sure, but it doesn't do much else that needed doing. All the classes are still in the same tiers, with maybe one or two exceptions; wizards and clerics can still break the game, fighters and monks are still craptastic. They supposedly either nerfed or buffed Power Attack, and nobody can quite come up with a straight answer which it is (as far as I can tell, they nerfed it for high-optimization purposes and buffed it for low-optimization purposes, which is good). Moreover, they did a lot of pointless little changes that are easy to lose track of, because they're so minor and there's so little reason for them.
So the idea that rings truest to me is this: treat Pathfinder as Unearthed Arcana II: a set of rules variants to pick and choose from at your leisure, applying some to your game and ignoring most.
And the rule change that seems biggest, and which I've most frequently heard praised, is that Pathfinder lets you cast infinite level 0 spells (known as cantrips if you're arcane, orisons if you're divine).
So, let's consider introducing that variant into our game. What do we need to consider?
Well, obviously, given a minute or two between encounters, it will make it possible to heal back to full health between each combat, because clerics can cast Cure Minor Wounds to heal 1hp of damage as many times as they want. But, oddly, the the things that have been sending the PCs back home in my games have mostly been ability damage recently, rather than HP damage. So this may not even have any effect. If it does have an effect, it will be to mitigate the effects of the 15-minute adventuring day at low levels, where HP and healing spells are the resources you most need to conserve. (It does nothing for high levels, where high-level spells are the only resource that matters, if you let your players get away with a 15-minute adventuring day at that point, which you shouldn't.)
Moreover, blaster casters can still contribute, albeit not much, even after they've used up all their spells. Acid splash, ray of frost, and other such level 0 spells let you keep doing damage, 1d3 points at a time.
I was worried about letting, say, a cleric cast any level 0 spell ever at will. But I consulted how Pathfinder does it, and it does it exactly how I was already inclined to do it: it's not just "infinite level 0 spells". It's "casting a level 0 spell doesn't unprepare it (for prepared casters)/use up a spell slot (for spontaneous casters)". Which is to say: a prepared caster can prepare a number of level 0 spells, and then use those spells an infinite number of times that day. I'm much more comfortable with that.
The other thing that worried me: there's at least one sorcerer feat that lets you use up two spell slots to cast a spell one level higher. If you have that feat, this could give you infinite level 1 spells, which is much less permissible than infinite level 0 spells. I'm sure there are plenty of other effects that work on the same principle. So let's add this blanket sub-rule, which I don't think is specified in Pathfinder: "If you use a level 0 spell or spell slot to do anything other than cast that particular spell, it still unprepares the spell/uses the spell slot." (A side-effect of this rule, which I don't mind at all, is that clerics must use one of their orison slots to prepare Cure Minor Wounds, because spontaneously casting it with another spell will still unprepare that spell.)
What of metamagic (ab)uses? Well, if you apply most metamagic to a cantrip/orison, it doesn't count as a level 0 spell anymore, so this rule doesn't apply. If you prepare a metamagicked cantrip as a level 1 spell, then it obeys all the rules of a level 1 spell, including using up the spell/slot once you've cast it. But some metamagic doesn't change the spell level! Such metamagic is in theory so weak that spending a feat to be able to apply it to a cantrip an infinite number of times per day should be more than a fair trade, but I don't trust it. So: "If you apply a metamagic effect to a level 0 spell, casting it unprepares the spell/uses the spell slot, even if it doesn't change the spell's level."
Additional possible variant: level 0 wands have infinite charges, and possibly level 0 scrolls don't vanish once they're used. (There's no way to justify potions not being consumed when you drink them.) I'm not sure whether this affects the game more than the original variant does; probably substantially, with the rule of thumb that however broken magic is, magic items are even more so (given that any character can potentially use certain magic items to cast any spell they want).
Having playtested this rule variant for a few sessions, I'm inclined to suggest this change: it works for every level 0 spell except cure minor wounds (and repair minor damage, and any other spells that restore hit points). HP are intended to be among the resources a party should take care to conserve, and allowing infinite cure minor removes that vital element of resource management. This change is distressingly arbitrary (what in-game reason is there for every other orison and cantrip to work infinitely while only these two don't? A better solution may be infinite cantrips but not infinite orisons, which is fine until you get a party where the tank is a warforged), but probably necessary.