Ideally, I want sims to surprise me with their behavior. But not surprise me only because they did something that was bizarre or silly. I'd like them to surprise me in the kind of way that real people surprise me. That I'm not quite sure what they're going to do, but I might have some idea based on their personality and history. The sims as they are in this game are, imo, too predictable and narrow in their behavior, and there's little to no illusion of intelligence to speak of. I guess that's probably why I prefer to play with autonomy off on the selected household sim. When I play with it on, I feel like I'm constantly trying to herd them away from doing things that are mindless or silly.
But maybe more importantly, I just want sims actions to feel like they are leading somewhere. Nobody has a visible "Fulfillment" need, for example, which makes them like hollow creatures. They know to Sleep, but they don't have any need to yearn, to imagine, to help others, to be a part of something, etc. Apart from the extremely general Social and Fun, all the non-physical needs are nowhere to be found, or might be found in some capacity in mood buffs that don't lead much of anywhere. As a reflection of real life, this makes it hard to relate to them as characters and learn and feel from the stories that result.
Some will disagree, I know. Some find other ways to get story out of this and I have tried the "projecting onto sims" thing. It's interesting, but I don't have much creative energy for it by the time I'm opening up a game to play.
And I will say, to the credit of the sims team, there are clearly some on it who have listened to various problems about this and have tried to do something about it over the years, and it shows. I'm just not sure the core of the game is where it needs to be and that makes it difficult for surface improvements to be very noticeable overall.