For years I’ve felt that there are some societal structures hard-coded into the AD&D rules, in particular the class system. While it is certainly possible to bend and twist things around to create a campaign world that bucks these otherwise built-in strictures (Dark Sun comes to mind), it’s very difficult to do without either scrapping and/or adding classes from the standard mix. I think it is because of this that so many of the published campaign worlds (Greyhawk, Forgotten Realms) seem somewhat of a similar mold, and it should be pointed out that one of the most successful to buck that trend, Dragonlance, does so by precisely taking out classes (clerics) and shuffling around some of the powers of those that they retain.
Take, for example, druids. We are told exactly how many druids there are, the details of their heirarchy, the process for selecting their leaders, and so forth. They are deemed a sub-class of clerics, but are clearly practitioners of a different faith than the standard cleric, with spells not only having a different emphasis but a different origin as well. You can certainly make a world in which druids function differently, but they would most likely not be the same druids.
Thieves and assassins are in a somewhat similar boat; campaign worlds are simply expected to have Thieve’s Guilds, a Grandfather of Assassins, etc. Take them out (or change them) and the classes becomes either weakened (if there are no guilds; no support structure to counterbalance the forces of the local law) or at the very least changed. A “standard” AD&D campaign should have provision for druidic circles, assassin’s guilds, bardic colleges, etc.
The “typical” cleric is another case in point. Many others have written about the quandry of having priests of different deities having almost precisely the same powers. A cleric of Thor is pretty much the same as a cleric of Osiris, who is the same as a cleric of Lolth, down to weapons restrictions and spell selection. This is a function of the origin of the cleric class in D&D coming from the tales of Roland and so forth, and is somewhat mitigated by affording clerics of certain deities some bonuses or other changes (clerics of Trithereon being able to use a spear, for example) or custom spells.
But when it comes down to it, the cleric class as presented implies a generic version of the medieval Christian church. Holy water, holy symbols (which were, iirc, explicitly crosses in the original D&D booklets), the very spells themselves (with their clear inspiration from both Biblical sources as well as miracles ascribed to Christian saints) all bespeak of a watered-down Christianity in a somewhat medieval mold. By rights, a cleric of Lolth should look absolutely nothing like a cleric of Blibdoolpoolp, and yet there they are, each with mace in hand casting cure light wounds.
Later editions attempted to get around this problem, but ineffectively, at least to my mind. (Second Edition’s class kits were possibly the best attempt, but they have their own idiosyncracies.) The bottom line is that there are certain things implicit in the design of the cleric that point to all clerics sharing the same faith, and point to at least how that faith should “feel”. A “standard” AD&D campaign should, by implication, have but a single faith for clerics to belong to (which does not necessarily mean a single unified church; a campaign could certainly have its versions of the Protestants and Catholics).
Personally, I think the answer is to come up with unique classes for priests of each faith, but that could turn into an enormous undertaking (although in my own Greyhawk campaign, I have been giving some thought to how an “imam” class would work for the Baklunish lands as a substitute for Suel/Oeridian/Flan clerics).
These assumptions could well have sprung from the original Greyhawk campaign, or were incorporated into the campaign after they had been written down as rules; I am unsure of which came first. But someone wanting to put together a “standard” campaign needs to take these implications into account, or they will run into problems as your high-statted fighter who multi-classed to thief starts wondering where to find the local bardic colleagues, and when your monk gets high enough in level to start looking for the Grand Master of Flowers for a one-on-one…