Since I started this, I figured I should get off my high horse and share my own opinions. Sorry, but I have trouble mapping things out, so this is going to be pretty stream-of-consciousness rambly, and for that I apologize.
I'll be blunt: although there's some gems hidden in 5e's fluff - I really like that it's the first edition since AD&D to put the Flumph into the MM, even if I do think that I prefer Pathfinder's take on the Flumph a little more - as a general rule of thumb, I prefer 4th edition.
Quick personal history lesson: coming from rural Australia, with limited funding and unwittingly getting hooked on Warhammer Fantasy & 40K (until I realized years too late that GW was basically determined to run it into the ground), I never had a lot of a chance to bury myself as deeply into D&D as I could. I absorbed all that I could find online, including ravenously chasing after uploaded copies of official D&D stuff. I'm not a world class expert on a lot of D&D things, but I do know a few things.
Although I cut my official D&D teeth on the Forgotten Realms, the wider world beyond the small segments presented by Baldur's Gate and the Sword Coast never really drew me in - ironically, I found my imagination more excited by the vaguely 1e-inspired setting of the D&D cartoon, which was still airing in my neck of the woods when I was a kid.
So, needless to say, I found the Nentir Vale setting to push all the right buttons for me. Ancient, fallen empires, lost civilizations, a world of adventure, darkness and hope... the Forgotten Realms were wonderfully detailed, but, at the same time, they never really excited me to think about playing in - they were more a world for reading in than for playing in.
As for Dragonlance, my other major D&D insight? ...Don't get me started. The Draconians were a brilliant idea, and I did read some books for it that I enjoyed, and it does deserve credit for being both the first setting to portray Minotaurs as a civilized species and for giving us viable Half-Dwarf and Half-Goblin species. But, between Kender, Tinker Gnomes, and especially that damn backstory with the Cataclysm... no, I have no love for Dragonlance.
The Nentir Value spoke to me in a way I couldn't really recall any other setting doing so before.
The addition of Dragonborn as a core PHB, especially after having lost their rather awkward "Races of the Dragon" fluff, really was a huge upside to me. When I read the "Races & Classes" preview book, I found myself nodding along with the book talking about how dragon-men were an obvious niche to fill and just inherently cool - but then, I've never been a huge fan of the bog-standard "Tolkien rip-off" flavor of the classic races, and fond of monstrous humanoids as something more than just enemies to kill.
Actually, that brings me to something else: I love the way many classic races were tinkered with and redefined in 4th edition. Each step from Basic took the "classics" a little further away from just straight-up ripoffs of Tolkien's work, but 4th edition was the cleanest cut of all - and yet they all still felt "right" to me.
Dwarves... well, okay, dwarves didn't change that much, but they still felt richer and deeper to me. I also liked the shedding of Tolkienisms like bearded females (which was mostly something that people laughed at) or rarer females.
The Elf/Eladrin split made a huge amount of sense to me; the Elf archetype in D&D has always basically been two ideas stuffed crudely into the same space, ultimately contradicting themselves. I mean, we've got a race that is simultaneously described in a very druidic fashion and supposed to be incredibly talented arcanists, despite the fact that druids and wizards are traditionally enemies. There's a reason why a Dragon Magazine article on magical plants points out that engineering life in such a way runs antithetical to druidic beliefs. A clean seperation made both stronger, as now they could stand on their own.
Gnomes! Oh, where do I start with gnomes? I never liked gnomes prior to 4th edition; they were always just lazy amalgamations of dwarf and elf traits, with no clear identity of their own. The closest we got were the Tinker Gnomes of Krynn, who were, like the Kender and Gully Dwarves, essentially a disastrous attempt at a "comic relief race". 4th edition was the first time I actually saw gnomes having any sort of value.
Half-Orcs dropping the default rape baby assumption was a huge step up. It was an ugly and limiting assumption, and didn't really add anything to the game.
Halflings finally dropping all vestiges of their hobbit ancestry and becoming new and unique was a huge leap forward, although I will admit that 3e had already made some pretty good steps in this department from their 2e portrayal.
Add in that the various new races of 4e tended to be so interesting - angels who forsook the heavens to live amongst mortals! Sapient fragments of a gate against the Far Realm! - and, all in all, 4th edition was a huge breath of fresh air in terms of racial fluff.
The settings outside of Nentir Vale... well, I can't really call myself able to play that game either way, seeing as how I didn't have the huge investment in Forgotten Realms, Eberron or Dark Sun that others did.
Honestly, I kind of liked the Spellplague and the loss of many of the more "novel-gluing" characters for FR, but I can understand people being annoyed at what they saw as a rather hamfisted rehash of the Time of Troubles.
For Dark Sun, though, I was pretty much in favor of everything that happened there. Tweaking defiling into a constant temptation for all casters felt so much more right than just making it another wizard subclass. As for the loss of mul sterility... I'll be honest; I've never seen any believable justification for there not being half-dwarves that didn't boil down to "but Tolkein never had them!" Beyond that, I also object to grimderp - a bit of darkness or edginess can do wonders, especially in the right setting, but making muls sterile and usually kill their moms in childbirth felt like just trying too hard to be edgy.
Speaking of darkness... I love a lot of the various lore we got for monsters in 4th edition. It's hard to really point to any one thing in particular, but a few things that stand out...
The "Playing Gnolls" article in Dragon Magazine was a huge thing for me. I always liked the idea of gnolls, but they've been traditionally one of the less represented "savage" races - despite their long history of being playable at the same time. This Dragon article gave them some of the best and most well-thought-out fluff they've ever had, and I was bitterly disappointed that it didn't make it into the Dragon magazine Annual, even when the goofy "Santa Dragon" mini-adventure did.
Shadar-Kai were awesome, going from a particularly assholish breed of fae from the rather dull Plane of Shadow in 3e to a race of immortality seekers who got what they wanted... at a cost. Essentially, they were playable, not necessarily evil cenobites, and that was an awesome idea.
Dragons were a huge upgrade, to me. When the designers decided to stick with the idea that "no monster actually in the monster manual should be Good, else what are your reasonable expectations of using it", that led to characterization of the dragons that I found more enjoyable than what they had before. Additionally, I have never really liked the core Metallic quintet being Gold, Silver, Copper, Brass and Bronze, as indicated in the threat I started about it - Iron and Adamantine just fit the core five so much better, and the subsequent metallic & chromatic dragons they added were awesome. Orium and Purple Dragons in particular were really cool.
Angels! Oh, I loved the change in Angels. The Celestials of old were... well, they tried, but they never really stuck out to me. Part of that was just my general distaste for the alignment, part of it was bad artwork (seriously, I say this as an actual fan of anthros: the 3e Guardinals were hideous), part of it was just a generally bland feeling they gave off. 4e's Angels, however, were everything I could have wanted in a standard Celestial. 4e's design paradigm of "if it's only Good, it doesn't really warrant stats in a monster manual" led to one of the most interesting and sincerely "angelic" Angels; aloof, powerful, and concerned only with the will of the gods.
Likewise, the elementals of 4e were a huge upgrade. Prior to 4e, elementals were... well, boring. For creatures as iconically fantastical as they're supposed to be, pre-4e elementals are just bland. I don't know who they are or waht they do, and I could care less. The most interesting things in the Elemental Planes were always "sub-elementals", like Genies and Azers and Salamanders and Tritons. 4th edition changed that. It gave us so many new and varied forms of elemental, all with their own distinctive forms and abilities, and really sold the beauty of the Elemental Chaos. Best part? You could still have your vanilla elementals, both as themselves and as the awesome "elemental soldiers of destruction" that were Archons.
Which brings me to the Cosmology... I've never really liked the Great Wheel. Why? Well, to be blunt, it falls into two reasons: grid-filling and blandness.
The grid-filling is obvious: we have seventeen different planes all based on desperately attempting to explore different conjunctions of Law, Chaos, Good and Evil. And the only reason we don't have nineteen to twenty-three planes is because TSR couldn't come up with any ideas that even they considered decent for "Neutral Good/Evil and True Neutral borderplane" and the like.
And that's just the Outer Planes! We've also got no fewer than sixteen Elemental Planes (four elemental, four paraelemental planes based on conjunctions of the former, and eight quasielemental planes based on conjunctions of the elemental planes with the Positive/Negative Energy planes), plus the Astral and Ethereal Planes, which have never really been defined in any way that made the distinction feel organic to me.
The blandness stems from that; we've got too many damn planes, so the interesting stuff has always been scattered sparsely over the whole overly saturated lot of them. To say nothing of how the Elemental Planes were pure "gotcha!" territory - you went to the Plane of Fire? You either fried outright, or you were presented with the "riveting" scenario of wandering through an endless 3-dimensional expanse of nothing but fire. Fire to the left of you, fire to the right, fire in front of you, behind you, above you, below you. Boring! And it was so boring that even TSR had to mix it up with the incorporation of some "elemental contamination", like floating islands/cities in the Plane of Air or the very existence of the City of Brass.
The World Axis, in comparison was organic. It was fluid, it flowed smoothly, it felt real, not forced into place to make up for everything. You had the classical but never really explored by D&D Worlds of the Dead and the Fae. You had the Astral Sea, which holds all the dominions of the gods together - and the dominions are brilliant, because they let you recreate and emulate the better ideas of the Great Wheel, like the clockpunk world-engine of Mechanus, without having to force them into fitting the framework of alignment. You have the awe-inspiring Elemental Chaos, with some of the most fantastic vistas ever seen in D&D and which would have been impossible under the old cosmology.
All in all... I loved 4e's fluff. The Primal Spirits, the world-building, the history, the cosmology, the way the races were expanded, the willingness to take a fresh look at old ideas... I could go on and on.
