I am trying to put together a list of the biggest changes in KULT: Divinity Lost compared to earlier versions. I will publish it here later on. But I would say that the single, biggest, change is the focus on the Principles. And how they make up the foundation of the illusion.
This is one of the strongest aspects of KULT: Divinity Lost and makes the whole game both more intellectual but, even more than that, spiritual in a gnostic sense.
This video is a nice introduction to the concept of the principles.