I like all my sims to be completely different, but each will generally have (strictly) 1 emotional trait, 1 negative trait and the last something really specific to their story / envisaged personality. Agree that negative traits add contrast to each personality, have never actually used Evil but Mean, Hot Headed and Hates Children absolutely! Low key negatives like Squeamish and Clumsy are also fun. The only other trait I have restrictions around is Child of the Island, limited to Islander natives and their descendants.
Also love incorporating the Parenthood inherited traits, if I haven't played with the sim from childhood I'll cheat specific traits based on their "backstory" which always keeps things interesting.
