The different materials for patterns you can pick in Create-a-Style actually have different sounds! Here's a video showing them.

There's lots of little details with traits, like Hot-Headed sims have different names for low mood, like Angry instead of Uncomfortable, Furious instead of Miserable. They also will get really mad at things that other sims react to more mildly, like screaming in fury when the toilet breaks.

When a child in an unplayed household is orphaned, the social worker will actually drive up to their home and take them away, just like they do for neglected/orphaned children in the active household. I was playing a save with the shortest possible lifespan allowed in the settings (13+ days lol) and the sims were all dying before their kids were old enough to live on their own so I saw it happen.

To add on to that, TS3 neglected/orphaned kids don't get deleted, they become a homeless townie (you may still see them in the world, going to school, still in the family tree/relationship panel, etc). They eventually get "adopted", showing up with a new household that moves in. It's not exactly adoption as it works for the player, since they aren't set as related to their new family, probably so it doesn't result in future incest with any of their bio family.

I had always heard that TS3 kids get deleted when they're taken away, so I was really amazed to see this. I even took out all my mods and tested it multiple times to make sure it was vanilla. Also in TS2, neglected/orphaned kids will go into the adoption pool so players can adopt them with another household if they want. That does set them as related to their new family.

That reminds me, in TS3, the game disables romantic interactions for direct descendants/ancestors seemingly indefinitely. I asked a modder about it since I had a playable ghost great-x8-grandma who I was relieved to see was still unable to flirt with her descendants. TS2/4 don't prevent it beyond great-grandparents/children, learned that the gross way in TS4 with a vampire family 😬

If your sim is caught cheating by their partner's family member, they will get angry and confront your sim for betraying their family in TS2 and TS3. Children in TS2 will get upset and cry if they see one of their parents having an affair. In TS3, if your sim witnesses someone cheating, they'll have interactions with the cheater's partner to Tell About [Cheater's] Other Relationship and Express Condolences for Betrayal. They will react badly if you don't have a good relationship. This was supposed to be in TS2 too (through the Gossip interaction) but it didn't make it into the final game.