Just have a look at the dev diaries. For me personally its the overhaul of pretty much all simulation engines (traffic, weather, water, wind, people etc.) and that they solved (apparently) the single thread problem of their traffic simulation. For me CS1 was bottlenecked when the cities became to big and the traffic could only be simulated on one core. There is a limit to that. But my cpu was otherwise idle. I have hope that this is now solved. Plus there is apparently no agent limit anymore. So a town of 500000 could in theory simulate all people individually, CS1 couldn't.