1upt of course. Probably the most common complaint, so let's just gloss over that for now.
I only have Civ6 on console. So I'm not as put off by the style with the goofy looking leaders and stuff since my intro to the series, CivRev, was also that way. And I love CivRev.
It's clunky and unless it's been drastically changed since I last played it (almost a year on now) the interface is atrocious. I had to look up how to clear queues at one point and it was completely vexing, no contextual prompt or anything and every time I tried to use intuition to figure it out I only got more frustrated as it just kept appending build orders or only inserting the new build and never replacing anything.
I find the map much harder to read at a glance. Call it lack of familiarity or blame it on the stylization, but I easily miss hill tiles still. No such issue with civ4, I can instantly identify terrain due to much more distinct profiles for each type. A floodplain is not easily mistaken for plain, jungle for forest, or a peak for a hill. In Civ 6, everything kind of melds together a bit and it gets frustrating.
Didn't really vibe with the district system. Fully willing to chalk that one up to lack of experience and it just being different. I just think simplicity of "building goes in the city. Period." is a lot less of a hassle than micromanaging the districts and their wildly all over the place reqs. Combined with tile improvement and juggling tile control it just seems like a little much.
Speaking of tiles, I think cities in Civ6 are unnecessarily sprawling. It's easy to control a lot of territory with a civ like Rome, utilizing it all seems overwhelming (again, lack of experience) but how crazy tall cities can go is just not as appealing as going wide, which I like a lot more in Civ4 which supports either. Didn't like the Tall bias in Civ5 either.
Still don't like city-states. Unnecessary clutter IMO. At least they are more interesting, especially since doing the right quest for the right city states can be quite powerful in a game so focused on investing in fewer, key units compared to Civ4.
I didn't really play it enough to bother to learn about how the GP work in Civ6. But it seemed like just another thing made unnecessarily more complex (civ6 is the one with the "lottery" right? or was that civ5? )
Things I really liked about civ6:
-Agendas for the most part. Makes the AIs more interesting. One of the things about Civ4 is that particular way that certain AIs behaved could define how you would react to that AI, and the situation you found yourself in with them instead of them just being cookiecutter rival players. Meeting Monty in iso or if Mansa is on the map at all can influence your entire game. I've played enough to form opinions of which AIs I like or don't like and why based on the way they play in many games. For instance, I hate AIs like Gilgamesh who are difficult to bribe but will freely attack others when he feels like it, compared to a complete psycho like Genghis who can be easily bribed away/counted on to go for a peacenik first. I see agendas as a continuation of that idea.
-The civic/card thing system. I think unlocking the new cards, governments being tied to your slots types/amounts, and the large amount of cards with various strategies was all extremely interesting, and probably the strongest part of the game IMO. The civic system in Civ4 is rather straightforward and loses luster quickly when you know which civics are better than the others and why.
-The tech tree and civics tree running in parallel is really cool. Culture is finally good for something in and of itself globally, instead of moving toward an objective obliquely through grooming certain cities. I thought this was handled WAY better than Faith in civ5. Eurekas are also fun as some of them are quite controllable (build X of this unit, build X improvements, etc) and not only just the random map-based stuff that feels way too much like huts in Civ4.