This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Pacific Rim
Mako's flashback to Onibaba's attack. Put yourself in her shoe(s): Your parents, and presumably everybody else you know, are dead. You are also alone in a devastated city. You turn a corner, and suddenly you find yourself face to face with the 188 foot Giant Enemy Crab, the one responsible for all of the above. Face to face leads to eye contact, and then it starts closing in for the kill...
It demonstrates something else, too. Here's this giant monster, so much larger than a girl no older than ten years old, who is alone out in the middle of the street. There is absolutely no way such a small body would make an efficient meal and the girl presents no threat to it. Yet it chases her with all the zeal expected of a Super-Persistent Predator. It goes to show just how completely ruthless the Kaiju are and how dedicated they are to killing every human they find.
TheDrift. In and of itself, namely the process which one entirely falls out of sync with their partner and reality before starting to 'chase the rabbit' out of temptation or lack of will, although the process in and of itself isn't that scary but rather the time and where it could take place since it's so spontaneous, like possibly in the middle the battle with a Kaiju...
Imagine being a rookie on your first real Drift, losing concentration for a moment, and being shunted into reliving the worst memory you have as a human being. Like Mako did. Now imagine being Raleigh, mind-linked and forced to experience it also without being able to affect a thing. And it's being done to someone you trust and like enough to have just shared your mind with.
Even worse: imagine being one of the test subjects who volunteered for the initial rounds of solo Drift research, knowing that practically everyone who's done it before you has had their brain fried.
Being targeted by a Kaiju? Bad. Being specifically targeted for tactical reasons? Very bad! Kaiju having a Hive Mind to the extent that a newborn immediately bolts toward you with the intent to eat you?OH GOD!
The deaths of Cherno Alpha and Crimson Typhoon's crews. Particularly the former, who we see drowning as the Jaeger fills with water, then being crushed by Leatherback. In the latter's case we see the pilots being crushed before the jaeger's head is ripped off its shoulders.
Raleigh was left a near-basket case for years because he was Drifting with Yancy when his brother died. Imagine what it was like for the last of the Wei triplets to die, who had to feel both his brothers' deaths!
Raleigh LOSING Yancy. Like the Mako example above, picture this. He lost Yancy to a Giant Enemy Crab/Threatening Shark hybrid, and he barely makes it out alive. He lost one Jaeger arm, and to an extent, an arm in the form of Yancy, his Jaeger is malfunctioning as he speaks, and land is far. He made it, but lost several things: His BROTHER and his former life. And you know what the worst part of it is? THIS IS THE BEGINNING OF THE FILM.
It gets worse: Yancy and Raleigh were still neural-linked when the former died. Raleigh literally felt the terror and agony of Yancy's final moments; he felt Yancy's death as if it were his own, and saw himself through his brother's eyes. Can you even begin to imagine what that must have felt like, what it did to his mind? Keep in mind that there is also a good chance that Yancy was eaten alive by the Kaiju.
Or how much worse it must've made Yancy's last moments, to die knowing his brother was sharing his dread and despair, so would have to live with that even if Knifehead failed to kill him too?
Please also consider that if Raleigh had died as well he would have had to go through it all twice.
Plus consider the reaction of the old man and his grandson that discovered Gipsy. This is proof the Jaegers aren't invincible.
Slattern's reveal. He is the first category five Kaiju and absolutely dwarves all the other Kaijus in size.
Then it goes and survives getting hit with a nuke.
Raiju is a category four Kaiju, very crocodilian in nature. Really the most dinosaurian of all the Kaiju. Nothing too bad, right? Well, its face splits open to reveal another horse-like face.
Otachi spreading her wings, and then quickly grabbing Gipsy Danger by her feet and hauling the robot higher and higher into the air...
Especially when you realize that she's effectively confirming that the Kaiju-makers won't be content with destroying the Rim cities, because she's ideally designed to fly over the Coastal Walls and attack the defenseless people inland.
The fact that Otachi can fly adds another nasty dimension to the Kaiju's continued evolution. Otachi can breed. This means that the Kaiju no longer require the Rift to start attacking in mass numbers. But that wouldn't be any good to a Kaiju that just lumbers about on land. But a Kaiju that could fly could go anywhere. Breed anywhere. And if one Otachi can produce an offspring, then each subsequent Otachi could produce an offspring. There would be Otachi's everywhere, each one capable of flying over any wall and liquefying any barrier that they come across with their acid.
Someone is driving to work across the Golden Gate Bridge and suddenly a gigantic creature ends an era, starts another, and most likely that person died without ever knowing just what that thing is.
The observant will note that one of the cars seen falling from the bridge is a police car with its lights flashing. A vivid illustration that the agencies we normally turn to for help and protection are completely powerless against what's coming.
As a corollary to the above, imagine that you've managed to survive a sudden and completely unexpected attack by a huge monster that seemingly came out of nowhere. After six days of brutal fighting (including, apparently, the use of nuclear weaponry), the military finally manages to kill the thing. It's over. Tens of thousands of people are dead, but the monster is dead. And then another one comes back six months later. And another one. And another. And another....
They don't just keep coming. They also come increasingly closer after the last. So when the first double event occurs, everyone is scared. Not to mention the triple event at the end, which goes hand in hand with the first Category Five.
Not only that, but it took three nukes to kill Trespasser. THREE. NUKES.
A fact made worse by the existence of similar disasters in real life: A man was in Hiroshima the day the Little Boy was dropped. He took a train to Nagasaki...the day the Fat Man followed. Statistically, someone who survived Trespasser may have been in another of the cities attacked by the Kaiju...
Who knows how much of the areas surrounding the Pacific are absolutely uninhabitable thanks to the Kaiju attacks? That, and the blue gloop that escapes from them as they rot contaminates millions of gallons of ocean water and makes entire areas unsafe for humans to live in even after the Kaiju are dead. And the political, economic, and social strife that must result from the mass migrations of people is only hinted at in the film.
As noted on the Fridge Horror page, it's not just humans the Kaiju are affecting. While they were created to destroy humanity, the impact they have on the Ocean's environment must be a thousand times worse. Coral reefs and Kelp forests cover thousands of square miles of the Pacific. A single Kaiju could cause irreversible damage just by stepping on them. There are also specific breeding grounds some species migrate to, such as California Sea Lions, that have probably become contaminated with Kaiju blue. Hundreds of species may have gone extinct during the Kaiju war, resulting in food chain depletion and starvation for millions.
The Kaiju are living organisms that need to eat in order to sustain themselves much like us. After Tokyo was attacked it is implied there were few survivors of the attack but the streets and surrounding areas are rather devoid of bodies and the assaults from the Kaiju seem to last for days. Guesses on what the Kaiju are eating?
The Kaiju shelters. Just the feeling of being boxed in, while giant monsters and mechas rampage 6 feet above you. And if the structure gives in, it's a perfect size for a mass grave. Or as Charlie Day's character said, a Kaiju buffet line.
The entire premise of the film. Think about it from a civilian perspective:
You walk down the street, looking for your friend, who you've organized lunch with at that little cafè by the bay. When suddenly, the water in the ocean begins to churn. What the hell's going on? Then, off the coast, you see the beast, rising like Godzilla. Its arms are the size of a small skyscraper, its body resembling a troll. It opens what looks like a mouth, and roars. It's a sound like you've never heard, like your skull is squeezing your brain. Then it grabs a cargo boat, and throws it like a stick towards the city. You dive for cover, and the boat cleaves through the building behind you. The monstrosity walks towards you, its footsteps hammering like a small earthquake. Every instinct is telling you to run like hell. But you're mesmerised with this beast, which towers over you by a mile.It looks down and spots you. That's when you turn, and start running. It roars again, and you trip. You close your eyes, and wait for your death. Suddenly, a loud thud sounds in front of you, and rises. A mechanical humanoid, at least 60 meters tall, stands before you. With both the monster and the robot occupied with each other, you scramble to your feet, and sprint as fast as your legs will allow.
The brief scenes we get of the "Precursors" creating the Kaiju. They don't just grow them in vats, they are literally assembled like cars on a production line. There's something rather disturbing about that being applied to a living thing, manufactured life form or not.
Forget the Precursors, look at the world on the other side of the breach. It looks borderline Eldritch, and that's without the aforementioned Kaiju assembly line.
If I remember right, the novelization mentions that the scars are burns from where the circuitry overheats and burns it's pattern into the skin (and the deleted scene where Idris Elba struts around half-naked shows his scars are very uniform and circuitry-like). Also, I think Raleigh's armor was torn open and blood spattered when he stumbled out of Gipsy Danger.
I just wanted to chime in that if you watch the opening of the movie when Knifehead is attacking you can actually see Raleigh's torso be burned by the circuitry on his suit shorting out. Later on he takes his shirt off and you can see the same pattern imprinted in his skin.
Though they're not intended to be sympathetic, some of the Kaiju die in drawn-out and gruesome ways that can be hard to watch. Leatherback gets its skull battered repeatedly and finally drops dead after its body cavity is blasted until it bursts open like a grape. Otachi has both its tail shattered and its venom sack forcefully ripped out, and lets out a chilling shriek as it's decapitated. Otachi's offspring is too premature to survive on its own and has nightmarish, bloodshot eyes. It's both sad and horrific to see that giant, half-formed fetus trying to kill humans, only to be strangled to death by its own umbilical cord. Then there's Raiju who gets bisected by Gipsy's Chain sword and Slattern who has his innards roasted by Gipsy's Nuclear Turbine. Of all the Kaiju, the one who had the most merciful death was Scunner who got vaporized by the thermonuclear bomb on Striker.
The fact that they're created solely as expendable weapons of mass destruction, driven to slaughter helpless humans at the bidding of a Hive Mind that extinguishes whatever rudimentary capacity for independent thought or feeling their constructed brains might've otherwise developed, makes their very existence rather pathetic.