Long time player here also. Some of my thoughts, contemplations and a brief game-psychological interpretation in the end.
I agree that Civ4 likely is and will ever be the best and most honest Civ experience there ever was. Only briefly mentioning those fantastic if not to say insane mods such as Rhye's and Fall of Civilization (the original one, not the overly modified modernized version not by Rhye which I don't like that much), Caveman2Cosmos, Fall from Heaven II etc.
I nonetheless sank many hours into playing Civ6. Not that I think that Civ6 is a particularly good game, but it isn't that bad either. Rather a mixed bag of some goods and many bads. And it can be enjoyable if you don't think too much about it while playing it in a more casual manner (or, at least, that is how I do approach the game).
If you search the forum you'll find several posts similar to "1000 things that are annoying in Civ6" and the like. I think that sums up the main problem of Civ6 pretty well.
There are layers and layers of mechanics ontop of each other in the game which don't work together so well, leading to many unresolved unnecessary quirks and inconsistencies. It is simply impossible to give a comprehensive overview of the many game-mechanic problems of Civ6 in this short post.
(The most famous example likely being the auto-path-finding algorithm, which is a reliable source of frustration in the game - it is preferable to not ever use it!)
And although Civ6 is very mod supportive most mods don't do very much except for some little additions or tweaks of the game. You'll find very little overhaul mods for Civ6 (are there any?). I think that is just a consequence of the many very confined mechanics of the game.
One of the design philosophies of Civ6 was inspiration from board games. But what works well on a small board becomes somehow overly inflated when in a computer game especially with a large empire/many civilizations in the game. The new game concept of districts is interesting and can be fun. But the game feels less like a Civilization game than any predecessor before. In the end there is a lot of micro-managment, especially a fizzly district placement optimization procedure. You may like it, or you may not like it. More of a grinding style civ game.
One can easily make fun of it: Instead "Civilization 6" a better name for the game might be "District 11".
In what Civ6 excels is the graphics, but this is of course mainly due to technical progression and some great artists that created it. If you like nice completed Wonder animations, Civ6 is the game for you (actually quite similar to what Civ4 had). That doesn't help the many flaws in game mechanics, of course.
Personally, I liked the Civ5 Wonders stils artwork style or the SMAC(X) wonder movies much better.
Some people may not be bothered by this but there are some other issues that especially annoy me when playing Civ6:
Civilization games were always competetive, but not as focussed on winning/domination as Civ6 now. It tends to be more about yield optimization than interesting strategic planning with respect to competing opponents. Therefore, I would claim that Civ6 has lost some of civ's "The journey is the reward" narrative gaming philosophy.
(Some of that criticism also applies to Civ5, which was a game where the main goal for a civ seemed to be "quickly increase some culture/faith bars and grab gold".)
Civ6 doesn't praise achievements of other players/civilizations, or, in other words, the development of civilization on a global scale. Wonders completed by another civ are accompanied with a nasty sound effect, so you will likely feel bad not having achieved it. That was very much different in Civ5 still, when wonders built by other civs were awe-inspiring too. The former civ games were much more about celebration of the evolution of civilization/humankind as a whole than the Civ6 installation is.
There are more such things that really make me feel bad while playing the game. For instance, the awful sound of agonized horses when a mounted unit dies. I think it is unhealthy getting used to such sounds. Even if it's "only" in a computer game. Here the game designers triffled with human conditioning.
So, Civ6 has definetively lost some of the original positive Civ spirit.
And just like these examples, there can be found many aspects in Civ6 that seem to have been created pretty carelessly. Civ6 is much less coherently meaningful in design than its Civ predecessors.
I would argue, mostly from the above reasons, the way the progession of Civilization game design over the past decade does reflect current tendencies of increasing narcissism in our society (from a game-psychological perspective). A sidemark from my observation.
