My thoughts:
-I know this is controversial, but The AI IS better. People keep saying it's not but I disagree. The tactical AI is the issue that most people have (I believe) because the AI can't do the same level of warfare a human can - we will just always be better. And the AI does make some questionable choices periodically, but it is more consistent (Artificial Intelligence just won't be perfect). But the grievance system has made the AI far more rational when it comes to diplomacy. The AI that I've experienced has better district placement, and overall better decision-making. One of my first games to get used to the mechanics I went back down to prince and actually almost lost a very close game. I was impressed with that.
-The diplo-spam after I get ONE diplomatic favor and my first THREE iron sucks. The trade spam makes me want to tear my hair out.
-I really like natural disasters as a whole - it adds another level of planning, decisionmaking, and consequence as someone who likes to play peacefully. Some games you get luckier with less disasters, others my river keeps destroying my farms before I get my dam up (but hey, more food). Side note: I still haven't been hit, or even seen, a hurricane. And I like building on the coast, dangit! Also, add earthquakes along continent lines and make Inca immune to those.
-Climate change needs a LOT of rebalance. It is absolutely ridiculous that building an infantry unit contributes so much CO2 to the planet. Often times, it happens too fast to get flood barriers up in a reasonable time. I have yet to have most of a city sink into the sea myself a la Indonesia in my Phoenicia game (that was both hilarious and terrifying), though I did lose about half of a large island once (but that turned an inland city into a city directly on the ocean, so it ended up being okay with me?). Also, it doesn't feel consequential enough. Sure, I've lost a few valuable tiles and the ice caps melted by 85% but that's...it. After that, you don't care anymore. You can deal with the slightly increased disaster frequencies relatively easily by that stage of the game. Where is my desertification? Snow and tundra thawing? Smog choking up major industrial cities and reducing housing without carbon recapture? Needs more risk.
Map generation: A+. The maps are so much better and I love just exploring my surroundings and planning my expansion and constructions. By far my favorite thing about the xpack.
-Railroads: feels like they contribute a little bit too much to CO2, but I won't complain too much about that. The increased unit movement is great - helped me respond well to some late game aggression, and it makes land/mixed land-sea trade routes worth a bit more. I wish they had more of an impact on industrial/agricultural uses since the railroad was revolutionary for those. Still, it was just nice to have them back - their absence was sorely missed.
-I think Tier 3 buildings could use a tad more of a buff, but otherwise they are better than they were pre-GS.
-The power system: I think they did a good job here with its implementation. It forces me to actually think about whether it is worth building a tier-3 building for a particular city yet because you can't just spam them - no power, slashed benefits. Also if you're relying on coal, you have to be careful when and where you build these too. It's not perfect, but a welcome change.
-Dams: Yes, I really think these were worth it. Other than being necessary due to the flood changes, hydroelectric power is great to have, and extra housing in key cities at stages where it's harder to get is nice for growth. Plus, it makes my agriculture towns look more complete and pretty
-Canals: They're fun - not a necessary addition at all, but I do like creating a canal network when I have large lakes that allow bigger industrial inland cities to become ports and supply my navy with ships. In my Sweden game, Helsingborg was my major seaport after I cut canals to the ocean through one of its tiles and another city's two tiles, and that was kind of cool IMO. The extra gold from trade is nice.
-Tunnels: are nice to have, and have actually opened up some late game city settling opportunities for me in valleys walled off by mountains that no one else has been able to claim yet. The teleportation aspects across the range are a bit op/ridiculous (ESPECIALLY in the case of the Qhapaq Nan being so early), but they are useful and fun. I approve. BUT the AI builds them on EVERY mountain they can. It's ridiculous. They need to be made less tunnel-happy.
-Ski resorts: YAY a use for high appeal mountains other than HOPING you can get four of them into a national park! This was a great addition.
-Flood barrier costs are too expensive when they are actually needed. If you get computers many turns before, they aren't an issue. But the ramping production costs make it impossible to save tiles sometimes as the sea level rises.
-Rockbands: early on and they are OP but they can also be your saving grace if you're in a close game (Space rock ftw!). I approve of this, especially because I already loved culture victory. Culture victory is a bit more challenging if you're not running away with it, and this makes it a bit more active of an experience. The downside is the heavy RNG reliance and no real defense beyond warring and murdering the bands.
-The new leaders/civs: All the ones I've played are the other reasons why I would absolutely recommend the expansion. So far, my favorite were Inca and Eleanor. Inca is a MONSTER when you settle in the mountains and make good use of terrace farms. You get massive and productive cities and can go for any victory you want really. Also, Qhapaq Nan are fun. Eleanor takes a bit of set up, but watching my friends just decide to join my civ without a fight is just rad. She was so much fun to try. Sweden was also cool (Nobel Prize for Physics is OP on so many levels), great for a culture game. Phoenicia is just fun - not a set of bonuses that makes you think they are OP, but I was able to claim any coast I wanted without fear of losing it on my continent. And I like the extra trade routes, and cothons are a fantastic harbor replacement. Hungary can construct their cities pretty fast when placed right, which is my favorite thing about them. Haven't played the others yet, but I'm looking forward to Maori and Mali. Ottomans is a bit warmongery for me so it's lower on my list (but of COURSE I want to try them eventually) and I'm apprehensive about a Canada playthrough.
-The refinements to R&F systems are also worth the price, especially Emergencies being so much better and more valuable.
-World Congress: overall I like it, but for me I end up having to be torn with the desire to be active in the Congress or to withhold all of my favor to HOPEFULLY attempt a diplo victory.
-Diplomatic victory: is just much too difficult to realistically win most of the time. I've pulled it off once. Sure, it was nice to see the AI team up to take me down a notch once or twice, but the WC is the main way to get those points. Aid requests and other alternative forms of the points don't come fast enough. And when you're too far ahead, the AI all repeatedly teams up to block you from more points/take them away and trading diplo favor - it becomes too gamey and a hoardfest of a currency that is too difficult to get, and that the AI can always exploit due to fewer votes and block voting being more valuable since votes scale in cost. A bit disappointing really.
-Can't speak for the new science victory since I haven't tried it yet.
-New units: it was nice having them flesh out the upgrade lines. The GDR is still ridiculous, but I do like how they were ultimately implemented with the upgrades coming from new techs, so it's less stupid than the Civ V version (that being said, I have seen the AI spam these and...that shouldn't be that easy).
-Future era: there are some cool goodies here, but so many of them come too late to have a real impact on my games. I wish there was a historic moment for the first seastead you build btw because don't you think floating cities would be a big deal?
-Cahokia mounds are awesome
i'm sure there's more I didn't touch on, but overall I would say it is a good/very good expansion that has nailed the one-more-turn feel for me that could still use some patching and refinements to be "incredible".
EDIT:
Right, resources. Overall, I like the new strategic resource system. But...make infantry maybe use your niter as its maintenance per turn instead of oil. Too many units use oil, and while mech infantry has a reason to use oil...foot infantry doesn't as much. i get the idea of making all melee (offensive) foot units use resources, but there is just so much demand on oil, moving infantry off of it will ease up that supply a bit while it still remaining late game's most valuable consumable in terms of units/power in many ways.