A big part of this problem is the emotion system. That ambient Happiness that plagues any room with half-decent furniture makes both trait-driven and incidental negative emotions totally pointless. You can see it in Toddlers. They're immune to ambient Happy moodlets, so their mood swings really matter in terms of their in-game behavior compared to older sims, who are affected by the scourge of Ambient Happy. The Neutral mood basically doesn't exist thanks to permanent ambient Happiness oozing from every chair and painting.
Then, there's the fact that traits haven't been integrated into the AI properly. Whether your sim is a Geek or not, the moment your sim leaves them alone, they'll likely make a B-line for the computer. Does your guest have the Active trait or any fitness skill? Doesn't matter, because they'll ditch in order to go for a random jog out of the Blue regardless. The away system also shows off the poor trait integration, as sims will be at random places all over the house doing totally random things regardless of the time or their traits...or even if the room they're in is theirs or not! It's like the game just rolls a D20 and lets it ride.
Then, there's no way to pick Favorites, there's no attraction system, every kid gets scared of the monster under the bed (sorry, but kids with the Evil trait should totally not be scared of the monster)...so much has to be done to improve sims' personalities in this game that I doubt that anything will be done at this point...
