Personality. Sims 2 had the most personality out of all the Sims games. By personality I mostly mean actions and animations of Sims. They could do so much more than in Sims 3 and 4, and had a lot more animations that really made the Sims look a bit more alive in a sense. There's also the fact that conversations actually meant something. You can literally see exactly what they're talking about, and what they're talking about always reflects their likes and dislikes from their profiles. However, I definitely do prefer the trait system, but it really feels like the trait system did erase some major personality.
Another thing is rotational play. Rotational play is only viable in Sims 2. If you try to rotate families in 3, you'll come back to see they all have new jobs, new marriages, or left town completely while you were away. The only way to stop this is to mod your game to double hockey sticks, which requires a LOT of techy knowledge to truly understand even how to make that work well. And in Sims 4, it's basically the same. Sims will flirt and make enemies while you're away, and just generally ruin everything you set up for them. In Sims 2, this thankfully didn't happen unless you made it happen by interacting with them. Yes, it's realistic for them to live their lives, and I personally prefer it this way since I like playing one family only, however occasionally I do want to rotate but it's disappointing that I can't in any other Sims game.
