What traits I don't like have changed with my play style. I used to loathe slob, but then after having it in a number of different sims in my 10 generation legacy, I got used to it and just made sure I had a neat sim in the house to clean up after them. It's now more of an annoyance to me.
Flirty, hopeless romantic, and inappropriate are traits that I love in certain games, but drive me nuts in others. Spontaneous sim drama can be really funny, but also frustrating. In a game I'm playing just for laughs, a flirty sim hitting on the grim reaper right after he takes her mom, and right in front of her husband, is pretty funny. But in a more serious toned storyline, I find that kind of thing frustrating. Inappropriate sims are bad for random come-ons as well.
I recall a game where I had a sim married to Cycl0n3 in Sunset Valley. They had been happily married for years, had two kids and were near elder, and he had some opportunity that involved Agnes Crumplebottom. I sent him to her house to take care of it, switched to his daughter for a minute, and the next thing I see when I check on him is him finding Agnes very alluring. Then I look at the relationship bar and see she became a romantic interest, I'm guessing because of an autonomous inappropriate kiss. He lost his eternally faithful moodlet, and because I didn't immediately do the "ask to be friends" and let it go, figuring I'd fix it later, the next day the wife found out and accused him of cheating. It took forever to fix their relationship to the point where the wife would even speak to him without getting angry, and I didn't want to use cheats, so I was worried I would not be able to prevent a divorce. I did manage to do it, but it really soured me on both the reputation system and the inappropriate trait.
Mean-spirited is another one that I don't like, because it requires me to micro-manage sims with it to keep them from being hated by everyone around them. Playing the Alto family once, I had to constantly watch Nick to keep him from getting both Vita and Holly to hate him within the span of a day because he was always autonomously going to pick fights with them, to a point that it did not even feel realistic. If he dislikes them that much, how were their relationships not completely red to begin with?