I realize this discussion may be rendered moot after today's announcement from WotC, but since they're only dropping hints and keeping everyone in suspense (as usual), I am going to indulge this topic even further.
A lot of the classic D&D settings do not diverge that far from the core of the game itself. Greyhawk, Mystara, Krynn, Nentir, and others traditional fantasy settings that easily fit into the basic mold of the D&D core. You could reasonably get away with a smaller word count to point out the flavorful differences and the few mechanical ones, sprinkle in some unique tidbits (like villains, monsters, gods, and magic items), and call it a day. Everything placed in the Forgotten Realms can easily be transplanted to similar worlds with just a few notes and minor changes. These are what I would call "core-friendly" settings.
On the other end of the spectrum are those campaign settings that break the mold. The three mentioned most often (and most recently) are Dark Sun, Spelljammer, and Planescape. What really sets them apart isn't how they affect mechanics, but more about their themes. Dark Sun should focus on survival in a hostile world with limited resources. Spelljammer should be about exploration and travel in wild space. And Planescape... is really hard to put words into one sentence, but it's a lot more than world-hopping! It's politics of the immortals, moral ambiguity, and a cosmic society operating on a level beyond mortal comprehension.
(Eberron falls somewhere in between. I truly know less about it than any of the other settings, so I'm not even going to mention anything more about it.)
If WotC were to support these non-Core settings properly... emphasis, properly... it should require more than one book with great concern for how it will appeal to people who aren't even interested in the setting to begin with. Does that even make sense? If you're going to give any attention to one of these settings, which do not fall under the traditional themes and expectations of the core game, then you should not be worried about making it fit better unless you want to change what makes it so different. If Spelljammer, for example, is merely a means to travel through space to get from one game world to another, then you only need to write a few paragraphs about how to get your FR characters to Mystara, Krynn, and back in time for the next DDAL storyline because you're not really interested in Spelljammer as a viable setting. More like Yellow Cab for D&D. Likewise, you can treat Planescape like D&D's version of Disney World(s). You can get there, but you're only just a tourist. I don't believe that's what many of us would sign up for.
And there is Dark Sun... okay, I'm redacting a lot of what I'm thinking on that front. I have a lot of ideas on how Dark Sun should be made into it's own product line separate from the core game for a lot of good reasons. That could be (and has been) it's own topic. Suffice it to say that all settings are not the same. Some would benefit from greater separation from the core game. I can only hope that today's announcement will indicate WotC is relinquishing some control over these settings to allow others to give whatever support and attention they feel their old favorites are lacking.