For me probably the quests Sims 3 has. I'm not that imaginative with gameplay and I get bored of standard gameplay sometimes and the quests really get me back into playing the game. Also rabbitholes, I love sending my sim to them and having a physical place where my sim works rather than them just disappearing for the day, and they make the world more interesting. Random one but I also miss the all in one bathroom, it was great for adding extra bathrooms to small houses.
What I don't miss though is the emptiness of it. It's so nice to go to bars and actually have more than 2 sims there, or to visit someones house and actually have them in. Also I miss the emotions system when I'm playing 3, I like them having actual moods rather than just happy and not happy.