First of all, some mythconceptions... EGG has posted that he designed the Cleric to be the "Christian" (Catholic) Knight. The Cleric isn't a Priest, he's an ADVENTURING Priest. Hence the heavy armor, and the original proscription against "bloodletting weapons" (although, as we ALL know, you can split someone's head with a mace as well as a sword). Hence the ability (in 3e, anyway) to cast
Cure spells spontaneously, turn undead, etc. These may not fit many religions, but fit Catholicism well enough (to my limited understanding, anyway).
The major PROBLEM with the Cleric, however, is the same as with the Monk; one is pretty much like another! This can be fixed, but that may not be what we really want... A Priest without heavy armor and spontaneous healing may make sense, but it is worse for getting PCs to play one, and yes, that is a concern!
So, we could have a "Cloistered Cleric" or "Priest" who doesn't have the armor, and doesn't adventure. We could also have an "Adventuring Priest" or "Cleric", but that doesn't address the real problem: one Cleric/Priest is still pretty much like any other...
So how do we fix that? NOT in the PHB!
Priests/Clerics should be deity-based. Their proficiencies, skills, spells, etc., should be based upon what they worship. More detail on the deity is required, first. Common, non-adventuring Priests (and the adventuring "Clerics") should be detailed therein, as well. Basically, this moves "Fixing The Cleric" from the PHB to
Deities & Demigods, or somesuch! Here's what I'd do:
First, detail the deity and worship, describing what that deity covers. Then detail the Priests & Clerics based upon that, making each one appropriate to the deity. Detail services, priestly garb, typical sacrifices/donations, services, items used, skills taught, codes of behaviour, weapons, armor, etc. Domains of spells need to be totally redone, with spells taught, spells which CANNOT be cast by Priests/Clerics of this deity, etc.... At this point, some examples will help. Before that, though, Domains!
Two Domains per Cleric may be "Balanced", but it is pretty dumb. A Cleric of a deity with many domains should have ALL of the domains that their master does. Maybe not at first level, but eventually. They should NOT be able to have others.
The problem that this brings up is "Balance". Demigods may have a single Domain, while a (so-called) "Greater" god may have many. That would encourage PCs to worship the "Greater" gods, and get more Domains... "Fine!", says I. Smaller cults will struggle to become established. Greater ones will have more power. This is typical of the real world's politics.
Now, Priests/Clerics of the Healing Domain should have spontaneous casting of
Cures, but PCs of Chaga, the Fire god? Nope! Likewise, Chaga's worshippers won't care didly-squat about a lot of other skills and stuff, but will focus on fire spells, being granted even Divine versions of the Arcane
Burning Hands,
Fireball, etc....
Quench, Cold- and Water-based spells will be anathema to them, etc. Weapons & armor will be the same, robes will be reds, yellows, and oranges, with the lowest levels wearing blues, and the highest white (following the colors of flames from coolest to hottest), and all having flame patterns. Temples and worship services will always involve fires, and sacriifices will be burned, melted, or smelted. Spontaneous casting will be of Fire spells, not Healing (which Chaga and his worshippers don't care so much about). Obviously, Chaga's only Domain is Fire, and his P/Cs don't get any others... His P/Cs get to use ANY flaming weapon without Non-Proficiency Penalties, however. They also lose Turn Undead, but gain Rebuke/Command Fire critters, and Turn Cold/Water critters, instead.
P/Cs of Mercury/Apollo, on the other hand, (regardless of whether or not they take the War Domain) automatically get proficiency with all bows, as he supposedly invented them. They have only Light Armor Proficiency, worshipping the god of Speed, and have Domains of Archery, Healing, Messengers, Music, and Speed. Probably Travel, as well, as messengers have to do a lot of that Spells of these Domains would need to be detailed, and casting would be spontaneous, for them. Turning Undead would be lost, and Perform would be gained as a class skill (Especially Perform (Stringed), for the Lyre). Medium/Heavy Armor Proficiencies (which slow you down) would be forbidden. To make up for all of the losses, certain abilities relating to the Domains of Mercury/Apollo would be added, as well as spells such as
Airwalk,
Feather Fall,
Fly, etc., as Divine spells.
Woshippers pf Arachnea wouldn't have it so good... Her Domains (as a Demigoddess or Lesser goddess, at best) would be Arachnids & Weaving. Her temples would be dark, hung with webs and tapestries, her priests garbed in silvery-grey, sacrifices might be made to spiders, or just donated to her P/Cs. She would teach Craft (Weaving), Climb, Jump, Rope Use, etc. Her P/Cs would be trained in Bola (ala the Bola Spider), Nets, perhaps spiked chain. Specialty spells for controlling or communicating with arachnids (which other P/Cs wouldn't have access to) would be available.
Spider Climb would be a spontaneous spell of the Arachnid Domain, but with no material component... (the MC for the Arcane spell is to EAT a living spider)! Killing spiders would be anathema, and a special ability, like
Arachnid Empathy would prevent Spider-like beings from attacking her Priests/Clerics.
Web would also be an Arachnid Domain spell, and her servants would have an immunity to all spider venoms... Turn Undead would become Rebuke/Command Arachnids. What kinds of armor? Spider-like Plate?
The problems with such a system are the work involved, and the loss of Turn Undead & Spontaneous Healing (Chaga's P/Cs can't, Mercury/Apollo's can, and Arachnea's can't). It is CERTAINLY easier to just deal with the Cleric... Just changing how Domains are done would do a lot (only Clerics whose deities have the Healing Domain can spontaneously cast
Cure spells). Having to memorize healing is exactly how it USED to work, so it can be done, again.
Anyway, the question is, is the work worth the result? Very specific Adventuring/Non-Adventuring Priests can be generated, each relatively balanced, and appropriate to their object of worship, but... can you imagine going through the entire
Spell Compendium, figuring out which Domain(s) each spell goes into, after going through the
Deities & Demigods to decide which Domains are needed?
What spells does poor Arachnea get for her Weaving Domain, anyway?

Does she get the Transformation Domain?
Baleful Polymorph?
Perhaps an easier way is just to bump the Cleric, leave the armor & weapons as they are, allow all of the Good-aligned ones Spontaneous Healing, add more Oomph to the Domains, and allow two to start, plus one per five levels, until the Cleric has all of the ones that their deity does, make Domains include all relevant spells, not just one per level, and expand the "gods of Greyhawk" chapter to include some Code of Behaviour stuff, as well as alignment & Domain info...
It's certainly easier!
