If you prefer to frame it like that, cool. I co-created my group's last two homebrew settings, and was the sole DM for one campaign. I like to design worlds to accommodate as wide a variety of PC's as I can, and I leave the nature of those PC's up to their players.
Which is still a world building descision. You have decided to set up elements of the game world to support certain sorts of player characters. It's no different the saying, "I want gunslingers, so black powder and guns exist," and working back from there.
If someone wants to play a faithful, benevolent Dudley Do-Right priest, fine.
If someone else wants to play a sinister, scheming priest of the same religion, also fine.
And that goal or desire determines the answers to the questions I listed.
My job as DM is to challenge the players while they run the characters they choose to play. It's not my job to challenge them to play the characters they choose to play.
Seems like a meaningless distinction to me, but to each their own.
My relevant questions are:
What kind of character does the player want to play?
How can I help them play that character while simultaneously while maintaining the proper level of game challenge?
How does the way they play their character reflect back on/feed back into setting fiction as I wrote it? (ie, if someone decides to play heretic of a church I created, how do I situate the heresy in the larger world, and more importantly, want further adventures can I wring out of it?)
Those are completely different questions, not really related to world building. They're really more about running the game then designing a world.
My response to this is: as DM, don't feel you are required to judge/evaluate a PC's morals, just throw entertaining and challenging situations at them in response to where they go and what they do.
If a PC gains power as a result of a deal with a more powerful NPC, it's the GM's job to adjudicate that deal. If a Divine caster gains power by following the tenants of his faith, or a knight by upholding his oath of fealty, or a monk by following a stringent philosophy, or an infernalist by signing a pact with a devil I'd venture that it's generally accepted that it's the GM's job to decide how well the PC is living up to that deal.
My response to this is: what DM wouldn't want this kind of collaboration in their campaign? Look, I love the settings I've designed. I really do (I'm so vain...). But as enamored of my own imaginative output as I am, when I comes time to actually run the game, I'm more interested in seeing how the players run with/reinterpret/generally eff with what I initially created.
... I bet you thing this thread is about you. Sorry, coldn't resist. Anyway, not all GMs like collaborative world building. I personally don't have strong feelings on it, although when I'm designing a wold I hold final approval. Typically though, in my experience, most players don't have a lot of interest in it, especially on this level.
I'm assuming a set-up like 4e where alignment isn't used as balancing factor.
Most of my answers have there basis in my most recent play, which has been Fantasy Craft, but I've tried to keep it more generic then that. Alignment in FC is mostly a mechanistic and targeting element in the rules. It's up to the GM when building the world to determine the nature of alignments (including which ones exist and what they do), what they mean in society, how they interact with each other, if divine magic exists at all, etc.
And I'd argue that alignment, or rather religious dogma, does play a roll in 4e. The whole divine power source for instance. Or would you let a Cleric or the Raven Queen who started working for Orcus in play still claim he was a member in good standing with her church? Yes, the cleric's powers and such can be left intact, etc, just say their patron shifted to Orcus from the Raven Queen, but I'm addressing the non-mechanical aspects.
Aha... I think this explains our differences. I trust my players --which I should say are all old friends at this point-- will create interesting characters, so I give them "license" to create whatever they want. I've no need to police their character concepts, and am quite happy bending the details of world to accommodate them. I mean, for what does the game world exist, if not to house the characters the players choose to play?
Not really. You're assuming I don't have a similar relationship with mine. But as I said, by letting the players decide if they're living up to their churches tenants in a world where doing so gives them power (I'd guess almost every D&D world, Eberron and similar distant deity worlds are the exceptions here) means that to be consistant, you have to let them decide if the powerful NPCs they make deals with think the PCs are living up to their oaths, pacts, contracts, etc themselves.
Now, you like the more, mechanistic and distant deities ala Eberon. There's nothing wrong with that, in fact it's how my current home brew works. Regardless of why that descision was made for any give world, it's still a world building choice. You make it for maximum PC archetype access. Eberon made it to be different then the other campaign settings and to support a more pulpy feel (if I remember right). I made it because it fits in with the cosmology and metaphysics (or rather, the lack there of) of the world I've been building.