In various expansion packs, Sims can use varieties of social interactions depending on other peoples' life states, such as what to imply their mother was (Varies on some supernatural types), unique compliments, etc.

Various Sims have names that hint towards at least one of their traits, career, family, and in some cases, life state.
And yes, I made a guide that lists them back in November 2018.

Strangetown, which is one of the neighborhoods from the base game of the predecesor, is mentioned quite a few times in The Sims 3:
-Infectious Disease Researcher level in the medical career, along with one of the newspaper entries, mentions a 'Strangetown syndrome'.
-If a Sim throws away good food, they get a moodlet stating that there are children starving in Strangetown.
-When a Sim reaches the top level of the Political career, they may do the Aiding the Victims opportunity, which mentions Strangetown suffering from natural disasters.

As some people might know, Pregnant Sims can never do certain actions, including the following below:
-Use hot tubs (Various expansion packs)
-Change appearance (Base Game)
-Skinny dip (Late Night)
-Clean litter boxes (Pets)
-Wear a costume at a costume party (Seasons)
-Streak (University Life)
-Snorkel (Island Paradise)
-Scuba Dive (Island Paradise)
-Windsurf (Island Paradise)
-Become a vampire (Late Night) (Supernatural)
-Transform into a werewolf (Supernatural, werewolf sims only)

Surprisingly, it's possible to make lesbian and gay couples in The Sims 3. What else should I insert here?

In University Life, it is possible for a Sim to get killed by vending machines if they try to get free drinks or murphy beds when trying to lower them. This can be unexpected for players new to the University Life expansion pack.

The Unlucky trait, despite being negative, comes with one overpowered advantage for Sims with it: If they die in any way besides old age, they just get resurrected. Perfect for people who don't want their Sims to die, right?

This is not considered the most immortal Sim, though, hence what I said. The real most immortal type of Sim is a vampire Sim with the unlucky trait AND the Immortal lifetime reward from the Supernatural expansion pack, which can never die of old age.

In Supernatural, there are magical jelly bean bushes that give a random effect: A random moodlet, catching on fire, being electrocuted, or 1 in 100, death.

Sometimes, there are very weird causes of electrocution and fires, such as:
-Blowing out the birthday cake candles.
-Teen witches instantly completing their homework. (Supernatural)
-Angering the weather stone. (Supernatural) (Seasons)
-Bad brain enhancement machine outcome. (University Life)
-Daring someone to divide by 0. (University Life. And hell, this is the most obvious weird one to me.)

If you wondered how some Sims can do various things, it indicates that they have a hidden trait. Hidden traits are found in NPCs and unique Sims by career, traits and/or life state. Examples include:
-Makes No Messes: Found in Maids and Resort Maintenance Workers (Island Paradise). This is not to be confused with the Neat trait.
-Pyromaniac: Found in children of genies (Showtime) and firefighters. They can start fires without destroying objects, transmogrify and fruit into flame fruits, and survive a fire for three hours long.
-Immune to Fire: Found in firefighters, Bonehilda (Supernatural) and Genies (Showtime). Sims with this trait cannot be killed by fire, but can still be singed. They'll also autonomously put out fires, even faster than normal, even without the Daredevil or Brave traits.
-Rocker: Sims will autonomously rock out as an idle animation. Mainly found in Sims who are at least level 7 in the Rock branch of the Music career and those parented by the said Sim.
-Burglar: Found in Burglar Sims and Sims in the thief branch of the Criminal career. This gives them the interaction to sneak around.
-Aura of Leadership: Found at Sims in the max level of the Political career.

If you have Supernatural, it is possible to hire a skeletal maid who can keep your house tidy. Unlike normal NPCs, she is free of charge, but the cost of her structure still adds to the bill. If you have Pets, she can teach dogs tricks, which is surprisingly essential. She can also stop burglars, even without a burglar alarm.

Bonehilda will scare maids (Which will eventually quit after fleeing her), cowards and neurotics, even if they have the brave trait.

Very oddly, Bonehilda can still steal clothes from skinny dippers (Late Night).

Lots of new players will not know that curfew exists for children (10PM) and teens (11PM) at first. Teens that violate curfew will be brought home by the police.

If Generations is installed, a grounding will occur once the child or teen returns/is returned.

The Miner invention from Ambitions can result in things and even have tips, such as:
-While drilling, you can come across gems, metals, scrap, old furniture, or even a gnome of invention. There is a bug that the collected items will not be added to the said Sim's score in the collection book.
-Be careful when drilling past bedtime, the drilling noises can disturb and/or wake up neighbors without the Heavy Sleeper trait.
-This can result in a relationship drop with the Sims that come.
-Drill holes on lots with a police station can have another result: Freeing one of the station's inmates.
-Freeing a convict can result in the criminal hideout providing a monetary reward.
-If you get this result, there will be a 2 day (48 Sims hours) cooldown before you can earn the result
again.
-Lower weight, mood, and points in the Logic and Athletic skills can improve results.
-It is possible to get the Miner by typing 'testingCheatsEnabled on' and 'buyDebug on' into the console and going into Misc. Items and finding it.
-Hygiene will decrease when drilling the hole, travelling between drilled holes, and even when exploring beneath the hole.

Many traits have unique interactions and abilities, but one of the most diverse known to me is the Insane trait. I have made a list for it in another guide already. I'm too lazy to re-write it all here.

During the hours of twelve to three in the morning, there is a low chance for aliens to abduct one of your Sims. If this happens, the camera will move to your lot and the randomly selected abductee will investigate it, which cannot be cancelled. However, it is possible to bypass this by placing a fence close to the location of the 'mysterious anomaly'.

If your Sim gets abducted, they will receive a negative moodlet, but non-Occult male Sims have a chance to be impregnated with an alien baby, like in The Sims 2. However, unlike The Sims 2, you can choose to send it back to it's home world or let your Sim raise it.

If you wondered how to place things without them snapping onto the grid in Build/Buy mode, this is the way: Hold ALT (Or whatever it is on Linux and/or Mac) while placing things on the floor. Yeah, this even lets you rotate things in any direction.