You know, I'm not really sure I should be posting here, given I said my piece way back at the start... but, I thought I might as well add my own two cents on some things.
All in all, I just like the World Axis so much more than the Great Wheel. I'll give the 5e version of it credit for trying to make the Elemental Planes more interesting by adding the Chaos as a buffer between them and the Outer Planes, making them physically coterminus, and styling them more after Exalted's Poles of Creations, but... well, frankly, the World Axis just does it so much better.
Seriously, the 4e cosmology was great because it took all of the interesting ideas that AD&D had spread over its absurdly oversized Great Wheel (17 outer planes, 16 elemental planes, 2 energy planes, the astral, and the ethereal, for a total of 37 planes, and I probably missed one or two beyond the Prime Material!) and successfully compressed them to fit in an even more interesting set of just 5 planes. I mean, come on, you got to give it credit for that.
The Shadowfell also stands head and shoulders over the old Plane of Shadow for me because of a simple reason; it's nowhere near as monodimensional as its "basic" planes. The Plane of Negative Energy was basically the ultimate "Gotcha!" Inner Plane, being a featureless, empty void that sucked out levels by the second. The Plane of Shadow was essentially a mirror image of the material world, but if you turned off all of the lights. The Shadowfell is more than the sum of its parts... still dark, gloomy, creepy, and full of dead people, but there's a mythic feel to it. When I think of the Shadowfell, I think of the scenes from Disney's Night on Bald Mountain, where ghosts are rising from their graves, with a dash of Tim Burton's gothic works, like Beetlejuice and Nightmare Before Christmas.
One of the things I loved was the little tweaking between gods and their followers. We have Invokers, which are essentially divine sorcerers cum prophets, who draw upon the most fundamental energies of a patron deity, and we have Avengers, who practice esoteric rituals as, literally, "holy killers". And none of this is alignment based. You can have an Avenger devoted to Sune or Wee Jas or any other god of beauty who's out to kill all sources of ugliness. You can have an Invoker of Lolth or Lamashtu who is Good aligned and seeks to redeem her patron goddess. You couldn't have that in 3e or AD&D - although, to be fair, you can still kind of have it in 5e, as it at least maintained 4e's attitude of "mechanically enforced alignment sucks and is counterproductive for interesting characters".
I think the biggest reason I loved the Elemental Planes changeover in 4e was that it made so much more interesting creatures possible. In anything prior to 4e, I can't have my Primordial Blots - embryonic, sapient planets, just waiting to be kindled into whole new worlds. I mean, how awesome is that? I can't have my Diamondstorm Reapers, which are Air-Mineral Elemental hybrids that can rip you apart in a shimmering swirl of gale-force winds and diamong teeth, because the Air and Mineral Planes don't comingle. We had so many unique and interesting hybrid elementals, with both "pure" elementals coming later and the archons of 4e standing from the beginning, that I can't understand why their presence is seen as a detriment and not an advantage.
Similarly, some of the features of the Elemental Planes were just so incredible. The Riverweb was an enormous spider-web like array of rivers floating in midair. Gloamnull was a demon-haunted, noirish flying city full of genasi. Heck, even the City of Brass got some shiny new features to it.
Still on the cosmological scale, the Primal Spirits from 4e were an awesome addition to the pantheon of gods, elementals, fiends and faeries. In all honesty, I never really liked the druid; like the monk, it reeked of token culturalism, an almost obligatory "Celtic" addition alongside the monk's "oriental" addition, but whereas the monk filled its own niche as a bad-ass barefist kung fu warrior, the druid was just an awful jumbled up mess, not quite sure if it was some sort of wilderness wizard or a nature priest. What really made it seem like a tacked-on addition was when actual nature-god priests became a thing in their own right, leaving you wondering just what the hell was the point of the druid.
The Primal Spirits answered that. They finally presented an "Old Religion" that really felt different to just "the resident rural deities" of the bog-standard pantheon. They gave a flavor to druids that made them stand apart, rather than just feeling like they were given the barest of handwaves to explain it.
But monster lore also played its part in why I loved 4e so much.
I will admit that Volo's Guide fleshed out the individual giants more than 4e, but the Ordning still doesn't feel good to me. I loved their 4e fluff, where the giants are the weaker imitations of the titans, the life wrought by the Primordials themselves in imitation of the Gods. A giant is fundamentally opposed to the world of mortals because it carries within it a spark of that ancient time, when the world was raw and untamed, and it wants to shatter the laws the gods put in place to make it different.
The Slaadi... I'll be honest, if I ever thought about the old Slaadi, it was with a level of disdain. Not just because they were Chaotic Stupid incarnate, but because they couldn't even be interesting in the bargain! Modrons were Lawful Stupid to the core, it was the very foundation of who they were, but they still had an intriguing culture, and more importantly, they could be something more. Rogue Modrons were by their default fluff a little monodimensional, but still, there's a lot of ways you can explore individuality developing in a member of what was once a hive race. Poor little Nordom was one of the most awesome characters to come out of Black Isle's D&D games. But the Slaad? They were never anything more than "I'z randumb! Iz funny!" The 4e version was... well, alright, I'll be honest, they're still not the most interesting of races to me - I find their niche pretty amply filled by foulspawn, aberrations and demons, thank you - but it was still a step up from the Chaotic Stupid parasitic frogs of 1st edition.
Gnolls... I think I went into this back on the first page, oh well. Playing Gnolls gave this race, which has been around since at least The Orcs of Thar, and playable throughout its history, one of the deepest and most interesting writeups they ever got. Torn between the beast and the demon, lured to evil but not incapable of salvation, creatures of the wild but not necessarily savage in their nature. It gave them a flavor all of their own and let them finally be workable as "monstrous adventurers" in a way that orcs, goblinoids and even minotaurs had been before them. 5e reducing them to little more than empty shells filled with Yeenoghu's hunger was an atrocity against their 4e fluff.
The Shadar-Kai were an incredibly interesting race from surprisingly stale beginnings. I mean, let's face it; the 3.5 Shadar-Kai's fluff, from their "Ecology Of" article, is that basically they're fairies who migrated to the Plane of Shadow to get away from the icky humans, found it backfired on them, and vowed revenge on humans because they're self-righteous pricks (you know, a lot like 5e's Tritons, but then I think they've always been that way), turning to self-mutilation in order to preserve their own existence. In 4e, what were they instead? Why, humans who sought immortality, and got it... at a price they didn't expect. But did they start moaning and bitching about it? Nope! They got up and embraced it, because carpe diem, baby! Better to live fast and hard, because dying in a blaze of glory is better than fading into nothing. The 4e Shadar-Kai are awesome and work wonderfully for a planar race, even if they do get a little Cenobitish in some interpretations..
Dragons! I loved what 4e did with Dragons. All these editions, and the Metallic Dragons made no sense to me - so, they're supposed to be the Good Dragons, and yet, everything I read about them suggests they're just as arrogant and controlling as the Chromatics. Changing them to Unaligned really was a huge step up.
More than that, the switch-over from Brass & Bronze to Iron and Adamantine was a huge improvement. The Copper Alloy Dragons nearly really felt that different from each other, and only slightly from their Copper Dragon kin. Iron Dragons were great as a Metallic analogue to the White Dragon; thuggish, brutish, feral critters looked down upon as the black sheep of the family. Plus, really, Adamantine/Gold/Silver/Copper/Iron just feels so much more natural than the original writeup.
And Orium Dragons were awesome. I mean, serpentine scholars of long-lost civilizations, rebuilders of ancient ruins, how is that not cool?
I don't know if I can legitimately talk about the gods or not, but 4e had some really awesome god ideas. Torog, in particular, was incredible. The King That Crawls, master of the Underdark, the force that makes even Lolth tremble in her little webbed stockings. And the picture of him was just... eurgh! Horrific, but cool.
Those are all the thoughts that I've managed to gather for this little rant. I might come back again with a brand new one, but, for now, these help emphasize why I loved 4e's fluff and would have gladly taken it over 5e's.