I think the flavor of a good setting should be defined as much by what
isn't there as by what is. I've always been bothered by the kitchen sinkyness of Eberron, for example, and I think it gives up its chance to be a truly unique and flavorful setting in favor of being compatible with everything WotC ever publishes.
That said, I kind of like the idea of
starting out with a kitchen sink setting, letting players use anything from any book (as long as it doesn't look unreasonable mechanically) . . . and then gradually defining the world formally according to what's already been suggested organically. That is, if no PCs or NPCs are using psionics, then maybe psionics doesn't exist. If the first few missions revolve around undead antagonists, then maybe undeath is the primary threat in the world, and orcs and fiends aren't really a big deal. If the party's Ranger is the non-magical variant from Complete Warrior, and they never run into any Rangers that
do cast spells, then maybe all Rangers are non-magical. If the Cleric worships the idea of "justice" instead of a god, maybe there aren't any actual deities with names and faces, just vague concepts invested with divine power. And so on. Hopefully, you'd end up with a world that's unique, and full of all the things your gaming group is interested in, with enough elbow room to explore those elements in depth.
EDIT:
schporto said:
At the start the players and I sit down and we all decide what books to use. At that point it is kitchen sink. Any book can be added, or denied. After that though, things are mostly fixed.
Aw, hell. Beaten to it. So what kind of world have you ended up with, through this method? I'm really curious.