Jer said:
At a minimum you have to go out of your way to explain why the Baatezu of the Planescape setting are exactly like the Devils of the standard setting but with slightly different names and backstory.
You have to do that with
any setting which doesn't use the Great Wheel cosmology, though, and technically speaking
both of the currently published official settings - the Forgotten Realms and Eberron - fall into that category. Of course, that the Forgotten Realms have been separated from the Great Wheel is not an uncontroversial decision, but the fact remains.
Likewise, Dark Sun doesn't take place in the default cosmology - and back when they had to pretend that it
did for the sake of consistency, they had to make up awkward explanations for why it didn't fit with Spelljammer or Planescape.
Likewise, Dragonlance was never written to make use of the Great Wheel, and it was poorly shoehorned in there.
Likewise, Ravenloft has no meaningful connection to the Great Wheel, and attempts to define it within that context (including the imprisonment of Vecna within it) always detracted from its uniqueness.
I'm a huge fan of Planescape, but for me its coolness never depended upon its use as a "metasetting" encompassing all the published
D&D worlds. Its coolness was dependent upon its own idiosyncratic elements - everything that was made up for the setting itself.
I like the sound of these changes from the point of view of making Fourth Edition a more accessible, flexible game. We can always rewrite the flavour for a Planescape setting in the future, just as we have to do it now for Eberron or another non-Great Wheel setting.