I feel that it's on the same level of difference as the 5e version from its predecessors.
Pretending that astral dominions are in any way comparable to the 17 outer planes and 18 inner planes and 3 interstitial planes and 3 or 4 additional planes is just fundamentally disingenuous.
There are five planes (plus one un-place) in the World Axis. There are, particularly with 5e's kludgy insertion of 4e ideas like the Feywild, more than 40 in the Great Wheel. The Great Wheel puts ALL of those 40 planes on equal footing. The World Axis absolutely does not put Mount Celestia on the same footing as the entire World. Mount Celestia is at best one (small) region.
This would be like saying that a Forgotten Realms where Neverwinter, Waterdeep, Amn, Rashemen, and Calimshan are all infinitely-large Ravnica-style city-planes is in no sense more complex than the singular FR world. Of course it would be! Each of those places is an entire plane unto itself!
I was not counting Eberron planes (which are their own thing), demiplanes like Dread etc., or any of the internal divisions of the 17 outer planes. That gives:Okay lets list the 5e Great Wheel planes/Layers/Demiplanes. Infinite layers or unknown amount of demiplanes are just counted as 1, because they are or are potentially infinite. I came out to 104.
1. Material Plane
2. Feywild
3. Shadowfell
4. Wildspace
5. Astral Sea
6. Border Ethereal
7. Deep Ethereal
8. Border Elemental Fire
9. Border Elemental Water
10. Border Elemental Earth
11. Border Elemental Air
12. Border Paraelemental Ice
13. Border Paraelemental Mud
14. Border Paraelemental Ash
15. Border Paraelemental Magma
16. Pure Elemental Fire
17. Pure Elemental Water
18. Pure Elemental Air
19. Pure Elemental Earth
20. Pure Paraelemental Ice
21. Pure Paraelemental Ash
22. Pure Paraelemental Magma
23. Pure Paraelemental Mud
24. Elemental Chaos
25. Outlands
26. Arborea 1st Layer Arvandor
27. Arborea 2nd Layer Aquallor
28. Arborea 3rd Layer Mirthardir
29. Ysgard 1st Layer Ysgard
30. Ysgard 2nd Layer Mulspelheim
31. Ysgard 3rd Layer Nidavellir
32. Limbo 1st Layer Gith and Slaad Layer
33. Limbo 2nd Layer Susanowo
34. Limbo 3rd Layer Indra
35. Limbo 4th Layer Agni
36. Limbo 5th Layer Lost Gods
37. Pandemonium 1st Layer Pandesmos
38. Pandemonium 2nd Layer Cocytus
39. Pandemonium 3rd Layer Phlegethon
40. Pandemonium 4th Layer Agathion
41. Carcari 1st Layer Othrys
42. Carcari 2nd Layer Cathrys
43. Carcari 3rd Layer Minethys
44. Carcari 4th Layer Colothys
45. Carcari 5th Layer Porphatys
46. Carcari 6th Layer Agathys
47. Hades 1st Layer Oinos
48. Hades 2nd Layer Niflheim
49. Hades 3rd Layer Pluton
50. Gehenna 1st Layer Khalas
51. Gehenna 2nd Layer Chamada
52. Gehenna 3rd Layer Mungoth
53. Gehenna 4th Layer Krangath
54. Hell 1st Layer Avernus
55. Hell 2nd Layer Dis
56. Hell 3rd Layer Minauros
57. Hell 4th Layer Phlegethos
58. Hell 5th Layer Stygia
59. Hell 6th Layer Malbolge
60. Hell 7th Layer Maladomini
61. Hell 8th Layer Cania
62. Hell 9th Layer Nessus
63. Archeron 1st Layer Avalas
64. Archeron 2nd Layer Thuldanin
65. Archeron 3rd Layer Tintibulus
66. Archeron 4th Layer Ocanthus
67. Mechanus
68. Arcadia 1st Layer Abellio
69. Arcadia 2nd Layer Buxenus
70. Arcadia 3rd Layer Nemausus
71. Mount Celestia 1st Layer Lunia
72. Mount Celestia 2nd Layer Mercuria
73. Mount Celestia 3rd Layer Venya
74. Mount Celestia 4th Layer Solania
75. Mount Celestia 5th Layer Mertion
76. Mount Celestia 6th Layer Jovar
77. Mount Celesia 7th Layer Chronias
78. Byopia 1st Layer Dothion
79. Byopia 2nd Layer Shurrock
80. Elysium 1st Layer Amoria
81.Elysium 2nd Layer Eronia
82.Elysium 3rd Layer Belierin
83. Elysium 4th Layer Thalasia
84. Beastland 1st Layer Krigala
85. Beastlands 2nd Layer Brux
86. Beastlands 3rd Layer Karsuthra
87. Abyss (infinite layers)
88. Daanvi
89. Dal Quor
90. Dolurrh
91. Fernia
92. Irian
93. Kythri
94. Lamannia
95. Mabar
96. Risia
97. Shavarath
98. Syrania
99. Thelanis
100. Xoriat
101.Far Realms (unknown amount of differemt realms)
102. Demiplanes of Dread
103. Demiplanes of Delight
104. Other Demiplanes
I was not counting Eberron planes (which are their own thing), demiplanes like Dread etc., or any of the internal divisions of the 17 outer planes. That gives:
1 mortal plane
2 reflections (Feywild, Shadowfell)
18 inner planes (4 elemental, 2 energy, 4 paraelemental, 8 quasielemental)
17 outer planes
4 interstitial/linking planes (astral, ethereal, wild space, elemental chaos)
Far Realm? Not sure if this was ported over from the World Axis or not
For a total of 42 (or 43) distinct planes, many of which (as you note) have multiple layers to them. Several planes are meant to be very important in their own right: the Abyss, Celestia, and the Hells are the primary examples, but Mechanus, Limbo, Acheron, and Carceri also come up a fair bit.
The structure, the symmetry, is of prime importance for the Great Wheel. Every space has a tight definition: a plane for everything and everything in its plane. Every possible cosmological question has a neat, tidy, scientific answer.
By comparison, the World Axis is intentionally loose. It has clear definitions only at the broadest categories. It leaves all other borders and boundaries intentionally fuzzy, indistinct. And travel between even its clearly defined parts can be subtle and almost unnoticed...at first. You walk through a mushroom ring and wind up in fairyland. You fall asleep in a graveyard and now you're awakening in the land of the dead. A door in the earth leads to a place where fire walks and water sings and mountains fly. Etc.
If you (generic) claim that the structure of the Great Wheel is mostly irrelevant, then you can squint and see an image of the World Axis. My response is that that claim is self-evidently false, and that the squinting reduces the comparison to near-triviality, effectively saying "well, they're both fantasy cosmologies, so neither is at all more complicated nor more involved than the other."
Putting things at the level of actual planes, as opposed to merely places inside a plane, is an extremely important thing to do. It signifies that that plane is important, that it is at the same level as the whole world and everything in it, it is an entire world unto itself. Putting something at a level below that is likewise also important. It says that those places are regions, or cities, or maybe even just a single building or ruin. A whole world has a host of additional expectations that a mere locale does not.
Okay lets list the 5e Great Wheel planes/Layers/Demiplanes. Infinite layers or unknown amount of demiplanes are just counted as 1, because they are or are potentially infinite. I came out to 104.
1. Material Plane
2. Feywild
3. Shadowfell
4. Wildspace
5. Astral Sea
6. Border Ethereal
7. Deep Ethereal
8. Border Elemental Fire
9. Border Elemental Water
10. Border Elemental Earth
11. Border Elemental Air
12. Border Paraelemental Ice
13. Border Paraelemental Mud
14. Border Paraelemental Ash
15. Border Paraelemental Magma
16. Pure Elemental Fire
17. Pure Elemental Water
18. Pure Elemental Air
19. Pure Elemental Earth
20. Pure Paraelemental Ice
21. Pure Paraelemental Ash
22. Pure Paraelemental Magma
23. Pure Paraelemental Mud
24. Elemental Chaos
25. Outlands
26. Arborea 1st Layer Arvandor
27. Arborea 2nd Layer Aquallor
28. Arborea 3rd Layer Mirthardir
29. Ysgard 1st Layer Ysgard
30. Ysgard 2nd Layer Mulspelheim
31. Ysgard 3rd Layer Nidavellir
32. Limbo 1st Layer Gith and Slaad Layer
33. Limbo 2nd Layer Susanowo
34. Limbo 3rd Layer Indra
35. Limbo 4th Layer Agni
36. Limbo 5th Layer Lost Gods
37. Pandemonium 1st Layer Pandesmos
38. Pandemonium 2nd Layer Cocytus
39. Pandemonium 3rd Layer Phlegethon
40. Pandemonium 4th Layer Agathion
41. Carcari 1st Layer Othrys
42. Carcari 2nd Layer Cathrys
43. Carcari 3rd Layer Minethys
44. Carcari 4th Layer Colothys
45. Carcari 5th Layer Porphatys
46. Carcari 6th Layer Agathys
47. Hades 1st Layer Oinos
48. Hades 2nd Layer Niflheim
49. Hades 3rd Layer Pluton
50. Gehenna 1st Layer Khalas
51. Gehenna 2nd Layer Chamada
52. Gehenna 3rd Layer Mungoth
53. Gehenna 4th Layer Krangath
54. Hell 1st Layer Avernus
55. Hell 2nd Layer Dis
56. Hell 3rd Layer Minauros
57. Hell 4th Layer Phlegethos
58. Hell 5th Layer Stygia
59. Hell 6th Layer Malbolge
60. Hell 7th Layer Maladomini
61. Hell 8th Layer Cania
62. Hell 9th Layer Nessus
63. Archeron 1st Layer Avalas
64. Archeron 2nd Layer Thuldanin
65. Archeron 3rd Layer Tintibulus
66. Archeron 4th Layer Ocanthus
67. Mechanus
68. Arcadia 1st Layer Abellio
69. Arcadia 2nd Layer Buxenus
70. Arcadia 3rd Layer Nemausus
71. Mount Celestia 1st Layer Lunia
72. Mount Celestia 2nd Layer Mercuria
73. Mount Celestia 3rd Layer Venya
74. Mount Celestia 4th Layer Solania
75. Mount Celestia 5th Layer Mertion
76. Mount Celestia 6th Layer Jovar
77. Mount Celesia 7th Layer Chronias
78. Byopia 1st Layer Dothion
79. Byopia 2nd Layer Shurrock
80. Elysium 1st Layer Amoria
81.Elysium 2nd Layer Eronia
82.Elysium 3rd Layer Belierin
83. Elysium 4th Layer Thalasia
84. Beastland 1st Layer Krigala
85. Beastlands 2nd Layer Brux
86. Beastlands 3rd Layer Karsuthra
87. Abyss (infinite layers)
88. Daanvi
89. Dal Quor
90. Dolurrh
91. Fernia
92. Irian
93. Kythri
94. Lamannia
95. Mabar
96. Risia
97. Shavarath
98. Syrania
99. Thelanis
100. Xoriat
101.Far Realms (unknown amount of differemt realms)
102. Demiplanes of Dread
103. Demiplanes of Delight
104. Other Demiplanes
The Far Realm would say hi, but its many squalmus tentacles are incapable of speech and its ten thousand lips are firmly planted on Lovecraft's behind.I'm in favour of any cosmology without the Feywild. Man has that been overused.