There are also traits with a low impact, like Vegetarian, Can't Stand Art, Good Sense of Humor, Dislikes Children, Easly Impressed, Excitable, Friendly, Frugal, Great Kisser, Heavy Sleeper, Light Sleeper, (Un)Flirty and Mooch.
Good sense of humor gives a boost to jokes, you can easily go up to a sim, make 3-4 different jokes and puff, you're a friend already, with barely any charisma. It's also the easiest way to complete the 100 jokes charisma challenge. I actually find it more effective than friendly or shmoozer for building up friendships.
Friendly, like good, prevents your sims from randomly insulting their partner/boss/partner AND boss by accident, they'll never be mean autonomously. Unlike good, it also comes with a bonus on certain conversation topics to make them more effective, which are also spread through most categories.
Easily Impressed and Excitable get you free moodlets for misc stuff frequently, i wouldn't be surprised if they also carried conversation options. Frugal gives a negative impact everytime you buy something expensive, and useless coupons the rest of the time.
Great kisser increases the chance of successful first kiss & other related kiss activities, also gives a heavy mood bonus for 4 hours for just kissing to BOTH sims, fairly good for a "kiss before work" and faster than brushing teeth.
Heavy sleeper lets you sleep with music, and sleep through roberies/fires/baby cry. Good if you have another sim or an alarm taking care of the emergency. No need to ruin the sleep of the sim that's going to work in the morning.
Moochs let you mooch money from sims, and mooch food from their fridges. You could go in their homes and live off your neighboors (and make money while at it) for the heck of it.
Haven't toyed with light sleeper or unflirty.
Flirty makes several flirt actions more effective, flirt actions not only take less sim time to do than say, friendly conversation options, they also build relationships faster. Flirty also fits like a glove to the heartbreaker and gold digger LTWs.
Can't stand art and dislikes children have heavy impact, that the impact is wholely negative is a different issue.
Also, if you count hidden traits, then Makes no Messes, Can Salute, Can Sneak, Can Apprehend Burglar, Immune to Fire, Rocker, Pizza Appreciator, and the [French/Chinese/Egyptian]Culture traits have a very small impact also.
First, hidden traits don't take up slots.
Makes no messes means no dirty shower/toilet/counter/kitchen, which means no negative moodlet or time wasted cleaning. Can salute gives you an effective greet that takes little to no sim time, can sneak & rocker are garbage, can apprehend burglar does 1/3 of what brave does for a free slot, Immune to fire does 90% what daredevil does for a free slot, pizza appreciator is miscellaneous albeit good meal buff isn't bad at all.
The different cultures lets you sing their songs, add an iddle to your sim (chinese would use a hand fan), autonomous sing song or sing song while cooking/shower, and a swift greet similar to salute. So basically, Can Salute + singing + random iddle/eat animation.
