Sadly, The Sims insists that "more is better"; whether friends (of the kind that advance a career, ahem), money, house size or personality points. Okay, Sims may start complaining if the house is so big that it takes them forever to get to the bathroom. But personality points are so much a "how good are you" thing that the Madame Blahbatfry crystal ball (supposedly wordplay on "Madame Butterfly", I was thinking more of Madame Blavatsky) will actually award points as reward, or subtract them as punishment.
Personality points aren't something that runs from 0 to wherever, though. They are presented, by the game itself, as the scale between two extremes, with a neutral point in the middle. The extremes are: Grouchy vs. Nice, Lazy vs. Active, Messy vs. Neat, Serious vs. Playful, Shy vs. Outgoing. Of course the "positive" qualities are used to name the categories - I speak of Nice points, never Grouchy points - to suggest that this is the desirable extreme to work towards. I say, and have found even in the game, that the most desirable state is no extremes at all.
To start with, I question the naming of two of the negatives. "Grouchy" isn't a characteristic, it's a mood. A FAQ writer has renamed it to "Mean" because "Grouchy" Sims are simply mean: they pester and torment, and this puts them in the sunniest of moods. It is Shy and Serious Sims who really get grouchy when rubbed the wrong way, as the red minuses over their heads show. And "Shy" is a misnomer for a character which, as the manual states, doesn't need much company. Shy people may be dying for company; it is fear that makes them retire. Solitary people simply don't want it and can be very pissed off if it's forced on them, and express their anger in a way a truly shy person wouldn't dare. So Mean and Solitary would be better names.
On to the supposed advantages and drawbacks of each extreme. The manual supplies me with the reasons why each extreme should be negative or positive depending which side of the spectrum is on: Messy Sims bring on roaches and lower the Room rating, while Neat Sims leave everything spick 'n span. Mean Sims find it hard to make friends, unlike Nice sims. Solitary Sims also find it hard to make friends since they don't initiate social interactions, unlike Outgoing Sims, and also are not as receptive. Lazy Sims lose Comfort points fast and hang about on the couch rather than exercising and gaining Body points. Serious Sims don't react well to jokes and entertainment, another disadvantage when socializing.
But the "positive" extremes have their drawbacks. Neat Sims neurotically clean up themselves, and after themselves; but not necessarily after others. I've had Neat Sims only wash their own plate and Messy Sims, once told to put their plate away, clear the whole table. Neat Sims will insist in wasting time washing their hands after every meal, even when the next action queued is taking a bath. I've had Neat Sims miss carpools because of their compulsive hand-washing, but not taking out the trash! Playful Sims will likewise waste much time playing, while Outgoing Sims will have their Social need plummet without someone to prattle to. Once their points and moods are firmly in the red, they will refuse to find a job, go to school or do a number of things that might raise their also plummeting Fun need. And both Playful and Outgoing Sims will earn the enmity of a Serious or Solitary Sim more effectively than Mean Sims would. Active Sims will also miss car pools, or opportunities to socialize and get their social needs up, because they don't want to come out of the pool; I've had Active Sims swim themselves to exhaustion. Finally, Nice Sims are spineless. Their niceness doesn't get them friends, nor do they sense when to leave the Solitary Sim alone; they just go "boohoo" whenever a not-so-nice Sim insults them, and they can get on the wrong side of a fellow-Sim completely with their eternal unsolicited hugs and compliments.
Now for the advantages of the "negative" extremes. I can't think of advantages to Messy Sims, except that they don't lose time cleaning up. They're a nuisance in the bathroom; to prevent repair bills, autoflush toilet recommended! But they don't seem to mind getting dirty, and don't whine for a shower so often. Mean Sims will find it superlatively easy to make friends if they are instructed to Talk non-stop, and their Outgoing points are high; and they won't pester people they like a lot. Paradoxically, they can get to like a Sim they can pester all the time! Two Mean Sims can build a fine relationship on Teasing each other, and the little head-bobbing dance they do while going "nyaah-nyaah" at each other is more fun to watch than the juggling trick. They don't boo-hoo when being slapped or insulted - they slap back! Lazy Sims, with their tendency to run to the couch and turn on the TV, are great entertainers, as guests will happily join them on the sofa, and they're not too lazy to chat; Active Sims tend to ignore their guests while enjoying themselves. Serious and Solitary Sims will at least not force themselves on guests, and make much less offensive hosts, although there's always the danger that they'll be offended by the guests. Their need points also don't drop as fast. For Serious Sims I'm not sure, as they can raise their Fun points by something as boring as reading a newspaper and so may accumulate a lot of Fun points unnoticed, but the Social need of Solitary Sims barely drops at all.
What the extremes do - whether "positive" or "negative" - is make
a Sim neglect some things in favour of others. So ten points in each
category (what one player called "the ultimate personality") means a
Sim that has to be closely watched, just as surely as zero points in
each category would. Zero points for Outgoing and Playful wouldn't
hurt at all, nor would zero points in the Neat category as long as the
household hires a maid. A Sim does not have to be Nice to make
friends (high Outgoing points are the best guarantee for relationship
building, high Nice or Playful points can be a hindrance) and no
Sim ever died for lack of exercise! The truly ultimate personality is
five points in each category, for the Sim that needs minimal interference
from the player.