Ryan Davis of GiantBomb had the privilege to sit down and interview Sims 3 producer Ben Bell:

They also have an article as well as a CAS screenshot of Ryan himself:

More significant to the actual gameplay than the aesthetic changes are the way your sim’s personality impacts the experience. During the Create-a-Sim process, you’ll choose a number of personality traits, ranging from charismatic to hydrophobic. There are traits that contradict each other, traits that complement each other, and there seem to be a pretty equal number of both positive and negative personality traits to choose from. This, I think, really speaks to how the developers recognize that there is no right or wrong way to play The Sims, and it can be just as much fun to play a neurotic screw-up as it can to play as some Eagle Scout. In addition to having a significant impact on how your sim will react to other sims and behave in general, the traits you choose end up determining which of the lifetime wishes you can choose from for your sim, which will serve as the overarching purpose of your sim’s simulated life. Your lifetime wishes, in turn, will inform the kinds of short-term activities your sim will benefit from engaging in. These personality traits aren’t permanent, however, and it’s possible to trigger a midlife crisis for your sim, allowing you to pick a fresh set of personality traits.