I'm also not convinced that this layout is really that useful for any purpose. What does a "border fire plane" do that a ring of volcanoes on the Prime can't do? Why can't the Prime have Basalt towers and Azers hanging out there?
There's also going to be a lot of doubling-up. 4e's Elemental Chaos was essentially Limbo, so it had Githzerai and Slaadi. It was also next to the Abyss, so we had elemental demons running around. So now we have Githzerai and Slaadi on the Elemental Chaos, and on Limbo on the outer planes, essentially doing the same thing in both places? And we have the Court of Stars in Arborea and the Court of Seasons on the Feywild? Shadowfell and Ravenloft and Border Negative Energy and also the Grey Waste?
when the books talk about extraplanar things, they don't have to be very specific. Okay, maybe the Demons are from Hell, but is this the Nine Hells a la Planescape, or Hell a la Christian mythology or Hel like Old Norse myths or Xibalba a la the Mayans or maybe Naraka from certain flavors of Buddhism. Maybe it's Hell as envisioned by comic books, or Hell like the Baha'i faith describes in which case, devils might just be creatures that were spiritually far from god -- humans, just twisted. Which one is it? Doesn't matter. It could be any of them. It could be ALL of them. It could be some of them and not others.
There are a lot of bits and pieces in D&D's planar lore that I quite like and would love to use. But I don't want to import the entire gargantuan edifice of Planescape just so I can have an adventure which involves Tartarus/Carceri. I would rather have each plane presented as its own thing, discrete from all others, and then we can have a chapter presenting cosmology options to hook them all together.
I tend to agree with these comments - in a "unification" edition it makes sense to present well-known planes in a self-contained fashion (The Hells, the Celestial Heavens, The Abyss, etc) and then assume that different settings/cosmologies/GMs will put them together for themselves.
4e had the barest gesture to this in its Great Wheel sidebar of the MotP, which explained how to recombine the 4e cosmological elements into a Planescape-y configuration. More of that sort of thing would be good.
I haven't seen any evidence that the existence of a "default" setting helps the game in any way. (There are games where it does, but I don't think D&D is one of them.)
a default setting dramatically lowers the bar for new entry into the game. If you have to build a world before you can play D&D, you've just turned off a whole lot of new players who simply don't have the time/energy/inclination to world build. There really are players out there who just want to sit down and play.
D&D should not come with a "Some assembly required" tag. You should be able to open the books, read the books and start playing. You should not have to invest dozens of hours developing a world before you can start play. Having a default setting gives you enough of a framework that you can jump right in.
I have quite a bit of sympathy for Hussar's point here, but a default setting doesn't require a default cosmology, given that the whole conceit of D&D is that you start at 1st level bound to the mortal world and only move into other realms once you have a few levels under your belt.
The idea that the game includes Hell(s), The Abyss, a Plane of Chaos, etc is enought default to kick things off, I think. Ie the game is helped by individual planer components, but doesn't need a default cosmological arrangement of them.
The issue of how much cosmological backstory there should be is trickier (and you can do this without metaphysical geography). 4e has a lot, and incorporates it fairly heavily into PC building, but is the only edition of D&D to do this, I think. For me that is core to the play of 4e (it takes it in a Glrorathan direction). Obviously many others didn't/don't like it. It seems to me fairly clear that D&Dnext won't have this.
I also think that linking all the worlds in some grand D&D metaverse isn't really that great a thing (either for Planescape or Spelljammer).
This I competely agree with. Metaversing just dilues the -verses. (And you can see that even in this thread, where we see non-Dark Sun products being held up as canonical from within the Dark Sun perspective itself. If someone wants to write a Ravenloft module with a Dark Sun element in it, fine, but that shouldn't create any default assumption that adventurers in Athas are only a mist (sandstorm?) away from a world of Gothic horror.)
Also, on border planes: I don't get them. If planes A and C need a border plane, B, then why isn't there a border plane between A and B, and B and C, and so on ad infinitum? Conversely, if planes can border one another without giving rise to an infitine regress of border planes, why do we need such comparatively lame planes as Dust, Salt, Ash, Mud, Smoke, Radiance, Minerals, and Steam? (Dust, Mud and Minerals all look to me like just parts of elemental Earth; Smoke and Ash like parts of Fire; Salt and Steam like parts of Water; and Radiance like part of the Positivei Material Plane.)
Lighting, Ice, Magma and Vacuum are more interesting, but Magma can still be part of Fire, and Vacuum part of the Astral or simply deep space. Lightning and Ice should be their own planes, given the preponderance of lightning and cold effects in D&D magic from the very beginning.