I like negative traits. I avoided them at first, but I find they can be pretty entertaining.
I don't like klepto either. For one thing, my other characters don't like it when their things disappear, so they try to avoid being friends with kleptos. But the other problem with playing a klepto is that they always get depressed if they don't steal anything, so it's hard to keep them happy. The little kids in my household love playing with kleptos, but I find them annoying.
I love evil characters. I love the sudden burst of evil laughter that they do, for no apparent reason; and how they feed off the misery of others. In my current game, I have a couple girls with multiple boyfriends that keep making their boyfriends jealous and sad. Then I make the sad people go "complain about their problems" to the evil guy. He loves it! The evil characters are also surprisingly good parents.
Mean characters are not such good parents. They get a "crush their dreams" interaction. lol I have never chose that interaction, but I think the parents did it themselves a few times. Sometimes their child would get these things that they hate being around "a sim that has caused them so much pain", and checking their relationship thing would show that their mean parent was the one who "hurt" them. The mean character I have right now has mean, snob, and self-absorbed (triple whammy!) She's actually been a pretty good mom and aunt, though, but I had a hard time getting her a boyfriend! Most of the guys didn't want anything to do with her! She got pregnant with a romantic, non-committal sim, because I guess that her negative traits don't bother him as much. But now she's also having an affair with an erratic sim, who doesn't care either.
Her snob trait is getting annoying though. She's always bored, because nothing is good enough for her.
Erratic is one of the most entertaining traits. Everybody should have at least one erratic character! They're always talking to themselves, and their moods change so suddenly. I just have one right now, but he also has clumsy and I gave him bodybuilder aspiration. His fitness is level 10 now, but he still falls off the treadmill! One of his girlfriends asked him to "just be friends" after her other boyfriend got her pregnant, and I got the message that they decided to cool things off. But he immediately started flirting with her again! Then she went into labor, and he started flirting with her in front of her new boyfriend, while she was going into labor! He just has no scruples!
Is goofball a negative trait? I didn't think so, but my romantic goofball bombshell is always walking up to guys in the bar and gym to make out with them, right in front of her other lovers, and then walking off. It's almost like she does it on purpose to cause mischief! It makes me laugh though, and my current evil sim loves it. He gets to feed off the misery she causes other sims. (He didn't like it so much when she started flirting with him in front of his pregnant girlfriend, while she was going into labor though.)