I may be using examples from games from multiple sims games, not just TS4
2 aspects I think should change:
1) There should be mandatory negative traits. Everyone has strengths and weaknesses. Its too easy to pick easy traits that make your game a breeze. There should be mandatory negative traits, like clumsy, awkward, gloomy, squirmy etc to add variety in your sims. At present TS4 has 3 trait lots, to account for this add 2 negative slots. I believe medieval has something like this? It'scalled a fatal flaw.
2) Some traits arent really "traits" at all. For example, why is vegetarian a trait? There should be a likes/dislikes slot to add things like this. It shouldn't take up a personality slot. Just like with "loves the heat" and "hates art" this doesnt say anything about your character as a person. These are just likes and dislikes. It should be in another section.
It can work in a space like this, likes and dislikes to add preference for food, weather, attitude about technology etc
Comments
Vegetarian, Cat/Dog Lover, Hates Art, Hates Children, Unflirty...
My biggest request would be to bring in a personality slider like in TS2, bring back favorites, and have the traits affect gameplay such as what @drakharis mentioned or even just like in the previous games where traits/personality would affect things like how fast or slow a Sim gained a skill or their ability to do one thing that another Sim with different personality couldn’t do.
What I think might also help is adding more slots for Traits and then changing how they organize and distribute the traits that we do have. In CAS, the traits are separated by Personality, Lifestyle, and the other one???Hobbies. What they could do is make the lifestyle and hobby ones more like favorites and interests in the previous games and add more Traits for the Personality category. And allow Sims to have more Personality traits in addition to the lifestyle and hobby traits.
But until traits actually affect gameplay beyond just moodlets, then it really doesn’t matter what they do.
I also like your idea about the negative traits, but, we probably don't need to force it like Medieval does (although, it is something I really do like about that game and adds an extra game play challenge element). As it is in TS4, I usually do try to make sure one of the sims traits are a negative one, not that it makes a world of difference in this version. If traits were ever more meaningful in this game, it could still be optional as it is now, a lot of us will pick them anyways.
Only allowing us three traits is hard when, as OP mentioned, some are more in the category of likes/dislikes
With how there are the options of being able to choose if your Male identifying sim can get pregnant or if your Female identifying sim can pee standing up, I think there should be a section there about "Vegan" or "Vegetarian". If you don't click either of those options, your sim will eat anything and be happy. That would take out the vegetarian trait that would otherwise clog up one of your three precious slots.
I know it isn't a trait, but I think with the seasons pack we should also be allowed to indicate which season a sim was "born" in. In TS3 you could choose your sim's zodiac, so if that were to transfer over to seasons, a Leo sim would be Summer, no matter how long or short you designate the season to change