All the little details that made sims appear more human such as cuddling on a couch or in bed, slow dancing, the way they would react to other sims actions. In the Sims 4 sims don't even react to being cheated on.
Realistic interactions like when sims would cook and it looked more realistic. In the Sims 4 my sims have bowls spinning around in the air, ingredients flying all over the place they throw knives around like it's some kind of circus show they're putting on.
Children that were actually like children. They had games they could play together and a good selection of items specially designed for them. In the Sims 4 children are like mini adults with school and chores. They have barely anything that adds game play specifically for them, they can't even play games together, it's just sad really.
Real babies. I hate hate hate object babies. The actual fact that they aren't sims bothers me so much.