The energy metaphors. The Monster world had hit 'peak scream' due to the cynicism of our world, though screams were also a form of renewable energy. Waternoose's solution was strip-mining screams, leaving a trail of damaged or destroyed children and hastening the end. Sulley's discovery that children's laughter had far more power than their screams was something like the Monster World discovering fusion; near limitless energy that wasn't in the least diminished by our world's cynicism. That most of the Monsters' jobs got much more pleasant afterward was a very big bonus.
Boo's Offscreen Teleportation. It's almost like a supernatural killer in a horror movie — fitting, considering that human children are basically the monster world's equivalent of supernatural killers.
Boo can move from place to place in a manner that's almost invisible. Who was assigned to be her monster again?
"I'm watching you, Wazowski! Always watching!"...considering that Roz was the head of the CDA, she probably isn't all that crazy...
It also explains why she grim-facedly pulls down the shutter over her booth when the CDA first arrive.
This line also takes on a different light when you find out about Randall's scream extractor conspiracy. Roz probably suspects that Mike's not filing his paperwork properly because he's in on it, not just because he's distracted/lazy.
Ever wonder why one of a pair of your socks disappeared when you were a kid? They probably got stuck to an unlucky monster like George Sanderson and nuked by the CDA...
The Scream Extractor becomes a great deal scarier when you imagine that it's not Mike or Fungus, but rather a kidnapped child strapped to a chair with a metal pipe descending on their mouth literally ripping the air from their lungs, either suffocating them or giving them severe brain damage. Keeping in mind that the targeted children mostly would not have even been old enough to fully talk yet!
Also imagine the many children that would have been kidnapped. Their disappearances would have been noticed. Just imagine how many innocent men and women would have been forced to spend the rest of their lives knowing everyone thought they were murderers due to Randall and Waternoose's actions.
Also, just think of what Boo's parents must have gone through when they went to wake up their little daughter the morning after she follows Sulley...
I always assumed that time moved differently in the human world and the monster world, such that by the time Boo got back, it was still the same night. All shots of Boo's room happen at night if memory serves.
It's possible she was gone no less than twelve hours her time if time is similar between the two worlds (it was nighttime in the monster world when Sully first met her). Boo is not of school age, so she probably doesn't have a set time to wake up though she will have to go to bed at around eight-ish if not earlier. Still doesn't make it less frightening for the parents.
Remember when Boo's laughter activates all the doors in the vault? Now imagine yourself opening up your closet door, not realizing there's a portal open, and falling to your death.
Consider what might have happened during the 15-minute sequence where Mike, Sulley, Boo, and Randall are riding around in the vault. There are millions of doors, so it's entirely possible that quite a few people would have opened one during that time, or even fallen to their deaths. Now imagine how much effort we would put into getting through one of those doors–and given the nature of what's through them, it's likely we'd come in all guns blazing.
That said, the majority of the closet doors are stacked behind other doors, so if they opened their closet they would see another door directly in front of them.
An in-universe example is the entire "Top Scarer" thing and the competition to earn that title earlier in the movie involving all the monsters going through many doors and whoever scares the most children is Top Scarer. Naturally, Sulley keeps his title. At one point, he even comes out of a door and simply says "slumber party", which boosts his points. This suggests that he scared an entire room full of children and it's Played for Laughs. Flashforward to later in the movie when Sulley accidentally scares Boo was his roar and "scary face" that he's forced to demonstrate. Boo's reaction is recoiling and sobbing in absolute terror. Then Sulley undergoes a Heel Realization of just how bad scaring has messed up a kid. That brings about the Fridge Horror; just how many kids has he left as scared and sad as Boo is now? Knowing that, even the "slumber party" moment becomes much Harsher in Hindsight.
It's also noted in Monsters University that crying is useless to the monsters, and dangerous because it's likely to get them caught by parents. As such, monsters are trained to use specific scaring techniques that won't make children cry. This may also be another trick by the company to get ethical monsters to accept an unethical power source — screams and tears are equally likely to bring parents running, but tears are more likely to cause the monster to sympathize with the target.
Early on, during a conversation between Cecila and Mike, she mentions she might get her hair cut. The thing is, her individual snake-hairs have eyes and brains of their own.
Made worse when you see them cringe in horror and shock at her announcement onscreen, then quite literally slump and sigh in relief when Mike told her he liked it the way it is.
This troper always assumed they would've grown back; it would've been painful, but by no means lethal.
Who says she doesn't have hair? We've never seen her without a shirt, for example...
As Sulley and Mike demonstrated, it seems to be quite easy to get back to the monster world simply by waiting by a kid's closet door until it's activated from the other side. So, assuming he wasn't Killed Off for Real by Crazy Shovel Lady, what's to stop Randall coming back and looking for revenge on Mike, Sulley and even Boo? Take a psychotic enemy who now has nothing to live for (since scaring as a profession has gone down the toilet and he'd be a wanted criminal anyway), who can climb walls, move silently and like lightning as well as turn invisible, and our favourite trio's futures start to look rather bleak...
Even assuming Mike and Sully just got ridiculously lucky and he can't make it back to the monster world, the movie implies they at least have a vague idea where the kids they're scaring live. He's got plenty of time on his hands (decades, most likely), and nothing else to do, he could conceivably hunt down Boo in her own world.
Monsters University implies that the Yeti was banished, but for a different reason. At the very end of the movie, Mike and Sulley are working in the mailroom. The two are warned by the Yeti that tampering with the mail results in banishment. The Yeti might have tampered with the mail and ended up getting banished. It's scarier that banished monsters are left to die in one of the most unbearable mountain ranges in the world, and the Yeti was lucky for being able to survive and building a home up there. It's also implied that all he eats is snow ... that is yellow.
Blink and you'll miss it, but at the beginning of the movie, Sully reads a file on a kid before going in the door. And in their commercial they mention that each child is matched to a monster that fits them. Now look at the number of doors and all the places the monsters are shown to go. So, basically, they have information about every child on the planet. And they use that information to scare them. Yeah....
When Mike goes to find Sulley after their fight, he has to venture into an arctic climate with the blizzard possibly still going. Given his smallness and lack of hair, he could have frozen to death if he'd gotten lost.