An AI.
Let's talk about it. The other day one of my Sims (I play rotationally) was talking to a townie. She confessed she was in love or crush on another Sim of mine, named Burke. Interesting.
How do I know this? While they were seated at the bowling alley she sat down and started talking about Burke. His picture appeared in her speech bubble (Burke wasn't even in the area and or on the lot), hearts appeared which in TS2 mean they love or are crushing on that Sim. My Sim also talked about Burke. His speech bubble showed a picture of Burke. They were discussing a Sim and the townie was in love with him. This doesn't stop there. Others will find out she is in love with Burke eventually. Someone will tell someone else. Did I have anything to do with it? No. But Burke might have met her at a club or something. Because in TS2 you have to be greeted by townies and or your own Sims sometimes. In TS2 she will not be deleted just because she is a townie. So, if I wanted to pursue this interesting tidbit of news I can.
The other day one of my Sims made the mistake of kissing another Sim of mine while out in public. She was dating a Sim named Chris. He saw this on the community lot. (I wasn't playing Chris at the time). He was not upset with her at first, but slapped the guy. Yeah, that makes sense since his girlfriend had not been the one that did the approach and kissing. She was on the receiving end of that kiss. However, when she kissed the Sim back, afterward, then Chris blew up and they instanstly broke up. ( a sign over their heads they were broken up).
He is still mad a week later, he often thinks of her and shows the break up sign over his head. I haven't gone back to him yet to see just how mad he is still yet. Out on community lots he is fuming. He will probably fight with that Sim later.
Wait, this doesn't end there. So, I move on to let him stew in his anger because I need to rotate to another family to do something important for them. So, that Sim who doesn't have anything to do with this gossips to another Sim about Chris and Carolyn breaking up. And why..they show the Sim's face in speech bubbles who caused the whole thing. lol
Wait, this won't end there. Evenually, more Sims will talk about this if I have them gossip. Select gossip. The gossip is real. It's not some random plum Sims talk about just so the player can imagine they are gossiping in TS4. The crush a townie has on one of my Sims is real, it's not some fake relationship the game decided to add an unknown Sim to a relationship panel like in TS4.
TS2 has an AI. TS4 doesn't. Townies will gossip. Chris will still be mad and remember it. Carolyn will be mad and remember it. And Burke will have many relationships to pursue. It won't be forgotten when they look at a happy painting.
And the next time Burke sees a townie out who has a crush on him, I already know it by watching conversations and can pursue it or not. That is just two examples of a proper AI.
ETA: Another example. Sims congratulate each other for things. Even getting to move into a house. (I guess they feel lucky)lol. Even townies will walk up and congratulate your Sim for a promotion, getting a house or whatever. What do TS4 Sims do or discuss? jibber jabber.