I generally agree with those who see the "inhospitable" planes of past editions as wonderful, interesting background, and disagree that such places need to be upgraded in terms of their accessibility to players. The examples already given are instructive: the ocean, the sky, etc. We don't need to fill the sky with castles or make (normal) water breathable so that we can have more square-footage for adventuring. The ocean and the sky are beautiful in their own right, and their inhospitable qualities are part of their mystique.
I also want to acknowledge the point that many have already made: yes, as a DM, you can change things to be back to the way they were, or make this or that alteration to the new cosmology. But this seems beside the point, because it's always true of any change made to the game or its assumed world. At some point we have to agree that some design decisions are better than others and that changing the game's default assumptions makes a difference to people.
Planescape was my favorite setting, and one meta-aspect of Planescape that made it feel believable (rather than "real"), was its complexity. The outer planes were colorful, complex, numerous, and mind-bending. They were realms of ideas and of contradictions. Even the "narrators" of many Planescape books struggled with grasping the multiverse, sometimes disagreeing with the standard assumptions about the world. Consider Planescape's "Faces of Evil", a sourcebook on fiends. One of the authors of the texts notes that he believes the Abyss is not actually made of infinite layers, but simply has an ridiculously large finite number. Similarly, when I read posts about how foolish Planescape is because there are an "infinite" number of demons, I wonder whether Planescape's very real hesitance to reify its cosmology is ever taken into account.
Here's a quote from Hellbound: The Blood War on the "infinite" number of tanari:
"But planar scholars argue that the baatezu aren't so stupid as to fight an unwinnable war. They say that Baator's commitment alone is enough to prove that the numbers of the tanar'ri are not infinite, no matter what the bean-counters might think. Instead, these graybeards offer two possible schools of thought..."
Basically, the planes are hard to figure out, and the scholars disagree. Most campaign settings simply don't work that way: the DM is told "what's really happening" (for the most part) and things go on their jolly way. In Planescape, the DM is told that the very fabric of the cosmos and the nature of its inhabitants and its wars are the subject of debate, not of certainty.
And that, my friends, was the beauty of the old cosmology.