Your Sim's Personality
Traits - What Makes Your Sim TickEvery Sim has his or her own unique personality, which is made up of ten individual traits. These traits will determine what kind of career your Sim will excel in, how quickly they'll improve their skills, how successfully they'll interact with other Sims, and in general will affect all aspects of their lives. Most of the Sims in the Story Mode will have pre-generated traits, which you cannot change, but if you wish to generate your own Sims, it's worth having a knowledge of how these traits will impact your playing experience.
Although there are ten traits in the game, each of these is paired off with another to form five axes along which your Sim is scored. For instance, the Neat and Sloppy traits form one axis; an individual Sim can be obsessive-compulsively neat, for instance, or fratboyishly sloppy, or somewhere in between. Again, you can't change this attribute once it's set; telling a Sloppy Sim to clean his or her house over and over again won't stop them from leaving the dishes on the table when you leave them unattended. The exception to this is that older Sims can encourage younger Sims to "improve" their traits through social interactions. Depending on the difference in ages between the Sims (an elder Sim is generally better at training than a mere adult), the power of the relationship between the two Sims, and the older Sim's own rank in the trait he or she is attempting to train, you can increase a trait by a single point with in two to five interactions.
Note that these trait axes are oriented so that each axis has one "negative" trait aligned across from a "positive" trait. Although the negative traits might have a poor real-world connotation, there are some benefits to be gained from having any of them, while even the positive traits have some disadvantages, and vice versa.
Sloppy/NeatIn the previous Sims game, it was almost always preferable to have a Sloppy Sim than a Neat one, as the primary benefit to being Neat was that your Sim would automatically clean up messes when they appeared around the house. Since maid service was fairly cheap (and still is, compared to the time it saves your Sims), it was better to let your Sims leave their trash all over the house, since it would be cleaned up every day anyway, and focus them on making new friends and improving their skills.
In the Sims 2, though, a number of changes have been made that actually make it somewhat less of a disadvantage to have a Neat Sim. They will still lose a bit of time when they automatically clean up messes (both those that they've made and those made by other people in the house), but the inclusion of the Cleaning skill means that they won't be spending this time doing absolutely nothing; each time they clean up a mess, they'll gain a bit of experience in Cleaning, which in turn can help them further their career goals. They'll also improve their Cleaning skill much faster than will Sloppy Sims.
There are also penalties for being Sloppy. In exchange for gaining more "me time" by never automatically cleaning up messes, Sloppy Sims will bleed Hygiene more quickly than their Neat counterparts, and will have a variety of somewhat disgusting habits, such as taking sponge baths, eating out of the trash, and eating food with the poorest table manners possible, which can lead to negative reactions from other Sims or even illnesses.
On balance, it's generally better to have a median score in Neat/Sloppy than to have a tendency to an extreme. If you're willing to micromanage your Sim's activities, then going all-out towards the Neat extremity will give you plenty of benefits; if you can manage to prevent them from spending all of their free time cleaning, then these benefits will arrive without much in the way of penalty.
Shy/OutgoingEvery Sim will have to interact with other Sims in order to advance in their careers and achieve their Wants, and the Shy/Outgoing axis determines their ability to do so, to a large extent.
Outgoing Sims are better suited to almost all gameplay situations than Shy Sims are, if only because they're less likely to suffer from negative interactions when they socialize with other Sims, which is one of the bedrocks of the game. This, alone, makes a highly Outgoing Sim better able to function in the game than a Shy one. Since being Outgoing will make it easier for your Sim to quickly make new friends, which every career path will require, and, well...that's pretty much automatically great, in and of itself. You can't understress the importance of socialization in this game, so a high Outgoing score is almost universally desirable. Highly Outgoing Sims will require more socialization to fill up their Social Need, which will also decay more quickly than it will for Shy Sims, but this is hardly a drawback, considering the amount of socialization you'll likely be doing, regardless of where your Sim lies on this continuum. Sims with high Outgoing scores are less likely to be perturbed when another Sim walks in on them while they're using the bathroom; they also have a good chance to strip naked when they get into the hot tub.
The only real exception to this rule is in the case of Sims with the Knowledge Aspiration. Since this Aspiration generally pops up Wants related to gaining skill points, your Sim will have to spend more time focusing on these skills, leaving him or her less time to spend socializing. Having a Shy Sim will let you invest the time required to build up these skills, without having to worry too much about your Social Need. Almost every other Aspiration will benefit from a high Outgoing score, particularly the Popularity, Romance, and Family Aspirations.