For me it's the memory system: That the Sims remember getting good/bad grades, having their first kiss, getting married, teaching their kids to walk, etc. And like @Cinebar mentioned, the gossiping (didn't remember to mention it before I read that comment). The sims actually "remember" the things they see or even hear and not only what happens to them directly. In TS4 all we get is the Stats table :D
Also the family tree not getting culled :s
I feel like the genetics were better in Sims 2 as well, that is for example being able to get more of a blend between the two parents for example if their skin tones differ. Although I do feel like that has improved in TS4 since I first got it. BTW I don't mean you get prettier kids because obviously TS4 is the winner there and has a lot more options for facial/body features.
I feel like Sims 2 was more about the gameplay and Sims 4 about creating pretty sims and beautiful houses but not so much about actually playing the game.
Can you tell I'm more about gameplay than building? B)
