About Sims 3? SO MUCH! Open world, cars, garage doors, spiral stairs, the cool gothic gable roof decorations, the futuristic decor, and the list goes on.
The main thing though, is how personable the sims felt. They actually felt like people, not just a semi-controlled bot working toward an ambition. Sims got out and just walked around, meeting other sims at random. Not like at a predesigned venue where the clientele could be predictable. I don't know, the whole atmosphere of 3 was different.
I think the sterility of 4 is why I mostly build now. But even that's on pause until some things improve and bugs get fixed.