It's not a particularly controversial statement to say that standard D&D has an implied setting. Where does this implied setting lie? Firstly, it is in the races and classes: we are in a post-Tolkenian fantasy universe. Secondly, it is in the equipment lists: we are in a high-medieval technological era with some anachronisms (mainly anachronistic lack of development of firearms). Thirdly, it is in the bestiary: we are in a world where there are lots of other humanoid races, lots of monsters, and also creatures from fairy-tales. Fourth, it is in the magic: this is a place where magic is relatively common, utterly practical (you might even call it mundane), and oriented towards adventuring magic-users.
And that's really all we need, isn't it? Only a blithering idiot could fail to run a D&D-flavoured game in a D&D-esque setting based on the core rules alone, nothing more. We've all done it.
It should be possible, then, for any non-standard D&D campaign setting to have absolutely no setting "fluff" whatsoever, being composed only of race and class descriptions and rules, equipment lists, a bestiary, and spell and magic-item lists. A zen approach to setting design, if you will, in which everything that a DM and players need to know about the setting (sufficient to run a game with that flavour) is communicated to them impliedly.