So, It has been a couple years now, so forgive me if my memory is a little imprecise… a lot this is going to be based on feelings I remember from the time.
The city districts were not fun. I was excited when I first heard about it during the previews, but I found them to be a frustrating puzzle. It was difficult to plan the city itself, and the bonuses and the numbers that drove those bonuses seemed hidden away somewhere, so I was playing somewhat of a guessing game. In the end, the specialization of the cities didn’t make me feel any sense of attachment to them, and the specializations themselves felt trivial over the course of the game.
Diplomacy was an absolute mess. Allies going to enemies overnight. Non-sensical wars being declared. Ridiculous demands. None of these things are new for Civ per se, but it was like they were turned up to 11, and it all felt very gamey. Maybe that was just me being spoiled from EU4 where the numbers are so apparent.
Eureka moments sounded like this great, organic function when I heard the previews… something to help you focus on different paths in the tech tree based upon your environment. In practice I felt like it did the opposite and almost put every game on the same tech rails.
Religion is the worst offender, and you’ve noted the part that drives me bananas - religious combat. It feels meaningless, and the carpet bombing of religious units was a complete and utter turn off for me. I struggle to understand how something so obvious has never been fix (unless it has?). It was a bit of a problem in Civ5 as well, but Civ6 just makes it front and center of your busywork.
But overall, for me it was a feeling of a game where i was pushing buttons and pulling levers, but didn’t feel any particular interest in what was happening. Civ5, for all its warts, had that for me. Maybe it is the cartoony art design of 6 (do not like), or the flippant quotes on the tech screens - or perhaps it was the direction to go in more of a boardgamey direction rather than simulation…but for me, as a guy whose played 25 years of Civ games, it just didn’t work.