I don't know exactly who started it, for me it began with Dyson's updated Turn Undead table, then came Delta's somewhat unrelated ruminations, and finally we got to Talysman's interpretation of turning as morale, so there was literally a whole "train" of cleric posts in the last few days. All of this made me think that I better share my take before someone else does. :-)
I've been fooling around for a while with a revised cleric class for my own "sorta B/X and sorta not" retroclone, and one of its key components is that clerics get something akin to the ranger's "giant-class enemy" or rather what became known as "favored enemy" in more recent iterations of the game.
Let's face it, while most religions like to think that they are "for something" they are almost always "against something" as well, and frequently they are against many more things than they are for. In a swords and sorcery setting in particular, there shouldn't be any shortage of religions, gods, whatever that are against a certain kind of "monster" or "creature" as it were. So gods of life and sun and all that should be against undead. Gods of nature and wilderness and all that should be against civilization (and anyone who stands for it: settlers, cavaliers, and rangers for instance). Gods of law and civilization should be against barbarians. There could be a god who's against wizards, a god who's against dwarves, heck even a god who's against other gods. And so on, and so forth.
With that in mind, I give clerics (who I choose to interpret much closer to the fighter/paladin end than the priest/wizard end) a favored enemy at first level. Depending on the background the player cooks up, there may be more favored enemies later. Some gods might be against several things after all. And everybody who can think knows that clerics are fanatics, maybe even bloodthirsty lunatics, who always bring down the hammer or sword or whatnot where they feel they must: on the heads of those their religions declare enemies.
Let's say the players encounter a bunch of wererats. There's a cleric of some god who is known for hating lycanthropes among them. The wererats might even know some other clan who's recently been wiped out by these "genocidal" clerics. What do you think will happen? First the wererats get a negative modifier to their reaction roll, making them more hostile than if the cleric wasn't there. Then, if they lose a few friends, they get a negative modifier on their morale check because they don't want to be the next clan to be wiped out.
The modifier will depend to some degree on the cleric's level of course, which is why this is similar to turning undead in the original game. What is very different, however, is that it works for all kinds of "monsters" as long as there's a suitable religion in the campaign. And as a fun twist, creatures with no intelligence are immune because they cannot understand the danger they are in. So no "turning" skeletons or zombies, not ever. Of course this assumes that the cleric isn't hiding what religion he or she follows, but most gods will probably expect that kind of boldness anyway.
Other combat-related changes for my cleric include a "no missile weapons" rule: your god expects you to fight bravely against those evil worms, no hiding in a tree with a sling or a bow. Also compared to "standard" clerics I slow down the spell progression, somewhat inspired by the Sohei class found in Oriental Adventures but not quite that extreme.
I really need to write that cleric class up in full sometime soon...