For me its the little, seemingly insignificant things I love about the Sims 4 game. The pregnant sims idle animations, autonomous affection from siblings, the fact that my sim will carry her coffee to the toilet (gross but funny) with her, the children who can't really help but try to shush or comfort crying babies anyway. My list could go on and on really because there are so many little attention to detail type things that I love about this version, but I think that's enough :D
The big things, the drag, pull, push options in CAS...the same options in Build mode...the ability to pick up whole rooms and move them and turn them in circles...I go back and play sims 2 and 3 because I still love them, but ultimately I always come back to 4 because of the little things, and also because of the bigger quality of life things.
I love that sims can carry on dinner conversation, chat with two or three friends at the same time, drink and eat at the same time... I miss the darkish humor from the earlier versions, but not enough to make the difference it'd take to get me to quit playing this one.