Alongside the release of Nubia, Civilization 6 has a "Summer Update" that makes a number of surprisingly major changes to the game.
There's a lot to sift through, including tweaks to many individual Civ's traits, but the most important ones fall into two categories: early-game expansion and the game's user interface.
Firaxis have decided to curtail the game's early expansionary tactics, a favourite approach of many (myself included) since it's so much easier to spread out early in Civ VI than it was in Civ V. The focus instead is now on growing your existing cities: settlers are now a whopping 50% more expensive, while there are various discounts applied to building districts, aqueducts and sewers.
That settler bump is going to kill me. I like the fact Civ VI lets you spread yourself out, as it makes more sense to me that most of the good spots for cities are gobbled up in the ancient era, and not the 1700s. Going back to a more Civ V-style focus on building up a key number of smaller cities is going to take some adjusting to.
As for the UI, there are changes and additional info now being displayed on trade routes, city state reports, combat previews and leader conversations. Diplomatic options have had a few changes as well, with detailed explanations now given explaining why you can't declare war on a certain Civ, and an added confirmation step included to make sure you really want to denounce someone.
Oh, there are AI changes as well. Firaxis say your opponents "will now better handle air attacks", plan their cities better and, most promisingly, they will "concentrate unit attacks on more threatening units rather than spreading out" during a city siege. We'll wait a few days to see just how big an impact that has on overall play, though. And it's a bummer that the navies aren't getting a fix, since they're still busted.
If you want to go through every single change, you can find them here.