Not a ton of information yet. What they've said so far is that cities will take up more space as they expand. They won't simply be a single tile anymore. Where you place them is more important as well.
Cities will have districts where specific buildings live. Like a campus district will have science buildings. You can't build buildings until you have the proper district for it.
Location also matters for research. Putting a city near water could give you a bonus to water based techs.
The AI for leaders change over the course of the game, depending on how you interact with them.
You can also combine units again. It doesn't sound like you can make stacks of death, but you could combine a Settler and a Warrior. for example.
The game retains the hex base of Civ V
It's using a new engine.
The lead designer of the game was lead designer on Civ V's expansions.
There is more stuff in the video. The video's creator, Quill18, does a good job of going over the press release. If someone finds a site that lists out all the announced changes, let me know.