I really miss having the likes and dislikes from TS2. I liked having sims interested in certain things and it affected how they acted or what they talked about and who they got along with. Having the magazines so that they could read and change and adjust these was just added extra dynamic. It really made my sims feel different though, like they all had their own minds. I liked it.
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.