Mainly: bigger cities, more simulated cities, prettier cities.
It also brought news of a release date: October 24th.
A press release offers more detail, however.

Cities: Skylines 2 simulated the economy such that your choice “have ripple effects across the city.”
The idea is that you might select a particular peep and then follow how your changes impacted their life.
Cities: Skylines 2 will launch October 24th and there’s aSteam pageif you want to see more screenshots.

NotE3 and Summer Game Fest 2023 is over for another year.