These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
One especially common point of interest is the Butcher. Since the only real image we see of him is a single memory, which may or may not even be true to what happened, he could be seen as anything from a simple family man who failed to properly explain the concept of raising animals for the slaughter to his impressionable son to an all-out sociopath.
Anticlimax Boss: The final boss of the game. You get big, beat it up until you run out of juice, you turn invisible, and then you wait to regain enough energy to turn big again. Even if your invisibility doesn't last long enough, as long as you turn it off and on before it runs out, you're still safe.
One could argue that this was done on purpose for the sake of symbolism, as Raz had just acquired exactly what he needed to bring the beast down: respect and understanding for his father and his intentions. The trope still remains regardless, though.
Best Level Ever: The Milkman Conspiracy. Just about every piece of dialogue is absolutely hilarious, and there's also next to no combat. Instead of fighting, you have to find a Paper-Thin Disguise to help you get past G-men who are also using paper-thin disguises.
Big Lipped Alligator Moment: The Nightmare fights. Originally they were going to be a running subplot in the game, but got dropped somewhere in development. The two that are fought are merely a leftover.
Edgar is practically something of a mascot for a number of the game's fanartists, though not necessarily as a subject - in general, the asylum inmates are rather well-loved in the fandom.
Loboto's role in the game's plot is bigger than in the game itself, but still, he's... pretty damn unforgettable.
Boyd Cooper's level is one of the most popular ones in the game, and with his simultaneously disturbed/disturbing and amusing character, he does look to be the most liked out of the Thorney Towers residents.
Genius Bonus: The Waterloo World level's background music is composed of the lesser known portions of the 1812 Overture.
Goddamn Rats: The exploding rats in the asylum are quite possibly the most annoying creatures in the game.
He Really Can Act: Richard Steven Horvitz as Raz. Right off the bat his performance is significantly more subtle than his otherwell-known roles, while still retaining his humor and distinct voice. But then there's a few scenes, like his opening speech or the scene where he finally makes up with his father where he has to be genuinely dramatic, and you'd be surprised how effective he is.
Hilarious in Hindsight: You get an achievement on Steam for listening to Vernon's entire story foreshadowing the asylum called "Wolpaw Says Thanks". This is very similar to an achievement in another, later Eric Wolpaw-penned game, Portal 2, where a very similar thing happens if you save the Prometheus/Oracle turret.
Either this or Harsher in Hindsight: The Phone Operator G-Man's line about mobile phones making land lines obsolete. This was before Blackberries and iPhones were everywhere.
The fight in Black Velvetopia against Tiger. Richard Horvitz would later oppose El Tigre, a tiger-themed luchador hero, as the evil Dr. Chipotle.
Les Yay: Seemingly subverted between Franke and Kitty (the most they do in-game is hold hands, something relatively standard of girls their age), and they refer to each other as "sista", but their Campster profiles list music and TV shows known for Les Yay, like t.A.t.u., Xena, and Ellen. Kitty also has "Dating, Serious Relationship (Women and Men)" listed under "Looking For..." Franke's only Interest is Kitty, and she's listed as being in an Open Marriage.
Memetic Mutation: Pretty much every line, but the gems given to us by The Milkman Conspiracy deserve special mention.
"OWW!" "MY FACE!" "HEY THAT HURTS!" "MY FACE!" "HEY THAT HURTS!" "OWW!" during Meat Circus's Escort Mission from hell.
And, in the jumping puzzle part of Meat Circus: "You used to like to play catch with me!" and your father's other repeated lines. Which you hear over and over and over as you try to make the same jump for the seventy-third time.
Porting Disaster: The PS2 port features numerous sound glitches and framerate issues and is probably going to crash at least once per playthrough, especially in the minds of the Lungfish or Gloria.
The Mac port is also really bad, with numerous sound glitches and occasional screen-blurring. Thankfully, Double Fine have a dedicated team working on patching the Mac and Linux ports, and most of the issues have been ironed out.
The Producer Thinks of Everything: During Sasha Nein's training, he tells you to defeat 1000 censors in order to receive the Marksmanship Badge. To generate these Censors, there is a lever with output levels to monitor in what frequency they appear with a gauge indicating from "1" to "SKULL". The game only progresses if you, being Raz, gets impatient and turns the switch to the "Skull" setting. If you go about killing 100 Censors without turning the switch up, the Censors stop spawning until you increase the level which eventually leads to you summoning the boss of the level.
Just to emphasize this further: When you just play through the game normally, you'll have Mr. Pokeylope in your inventory for about 10 seconds. Yet, every single person way back in the camp has a unique reaction to that item.
Just like with the items, every person has a reaction to targetable PSI abilities like PSI Blasts and Pyrokinesis!
The Scrappy: Clem and Crystal can be annoyingly cheerful to some.
J.T. is this to people from the American south (and especially Texas) who resent the stereotype.
The last fight totally makes up for the rest of the stage, though.
Five years after the game's release on Steam, a patch was made available that made the Meat Circus easier.
Also, the upper levels of the asylum. They're dark, have some tricky jumps, and are home to some really annoying enemies.
And The Race section of Milla's mind. Running the race is fun, but going through to collect all the figments is awful; it takes over twenty minutes to go over all the routes thoroughly, most of the figments blend into the track around them, and when you finally reach the end and realize that you're still missing a few you won't be blamed for wanting to throw your controller at the screen.
There will come a moment where you'll eventually have to begin grinding for the expensive cobweb duster, either by killing enemies for about an hour, or by doing the immensely annoying Dowsing Rod minigame where you have to nearly break your keyboard in half as you tap like crazy to rip out another arrowhead from the ground. It's even worse if you waited until the Duster was needed, as you'll have to attempt to find arrowheads in a dark camp full of psychic animals, meaning you don't even get the benefit of hunting for deep arrowhead deposits.
That One Side Quest: Collecting all the figments to get 100% Completion. They are hundreds of them in each level, they are 2D, they move, they fade in and out, they are neon which makes them hard to see in several levels, and there is no radar or anytihng to help you track them down. This is at its worst in Milla's Dance Party, where the color scheme of the level combined with the figments being heavily spread throughout the racing portion make it difficult to find all of the figments there without a guide.
Toy Ship: Raz and Lili, Elton and Milka, Nils and J.T. with Elka... Raz himself said it best, really.
Raz: Is making out all anyone ever thinks about around here?
Don't forget Bobby and Chloe albeit in a nigh impossible to find cinematic.
Ugly Cute: About half the characters in this game are sort of adorable, Shegor in particular.
Linda is pretty cute once she's not being mind controlled. What a magical lady...
What Do You Mean, It's for Kids?: Peoples' fears and nightmares in this game can be downright unnerving. It features a lot of people with childhood trauma, animals burnt alive, spirits of children who died in fire, a pre-teen girl shooting a rifle and, when cornered, throwing herself out of a window and then blowing herself up to kill her enemies and... well... Meat Circus. That said, the game isn't exactly targeted at kids. Avoid the Dreaded G Rating on main page.
Sasha's father can qualify too—lost a wife he dearly loved and drove his only child away, probably without ever realizing how. The heartbroken look on the poor man's face through the whole reel just makes you want to hug him.
Milla Vodello: Used to take care of children... until they all died horribly in a fire. Still has little bits of that nightmare locked up in her mind. While she seems to be all better now, many fans seem to think she's still haunted and deeply troubled by her past. Even if she's coped, it's still pretty darn tragic...
Ford Cruller is a particularly tragic example: all the different roles he plays in the camp, from cook to forest ranger to janitor, are multiple personalities caused by psychic damage inflicted on him by an evil psychic. The fact that Ford is the most adorable, nice, caring adult in the entire game makes it downright heartbreaking.
Dogen Boole: Small, runty, picked on by the other kids at camp, terrified of everything, barely able to control his own psychic powers, the first to lose his brain to Dr. Loboto, and even those who are generally his friends (Raz and Lili) still take occasional pot shots at his expense. While some are annoyed by him, others find him somewhat sad.
Gloria: While the rest of the inmates have somewhat goofy histories, Gloria's is genuinely sad: she spent much of her childhood at a Boarding School of Horrors, dreaming of the day her parents would arrive and take her home. While she shot to stardom, she abruptly crashed when she found out that her mother had committed suicide. Sure, she acts funny, but she is clearly deeply troubled.
Even more upsetting when you consider and combine the events of the "Tragedy" plays; her mother committed suicide because she never got a letter back from Gloria while she was in the Boarding School... because the letters she was sending never got sent by her agent.
Sheegor: Dr. Loboto's beleaguered assistant, she lives in fear of him and what he might do to her pet turtle, Mr. Pokeylope. He abuses her constantly, and she has a childlike mind. When you use Clairvoyance on her, she sees Raz as Jesus an angel, come to save her from her torment.
Raz: A temporary case maybe. When he starts to talk to Ford about how he thinks his Dad hated him and tried to distract him from his powers and/or kill him with endless training, it's hard not to feel sorry for him. This is temporary though, because by the end of the game we find out it was all a misunderstanding/misinterpretation. He's still cursed though.
Every kid seems to be a woobie to some degree, judging from Raz's speech and how they react to it.
Oleander. Regardless of how bad his dad might have been, he wasn't allowed to love the rabbits as a child, and was rejected from his dreams of the military for his height. You can't help but feel like he's a man with nothing but failed dreams that were out of his control, it's hard not to understand why he would want to take over the world.
Milka Phage. In the actual game proper, the player doesn't get to interact with her very much, due to the fact that she goes invisible whenever Raz shows up. But her Myspace account goes a bit further into her backstory. The reason she likes to turn invisible so much is that her mother used to make her because looking at her face upset her, due to the fact that she took after her father. She used to pretend to run away from home by going invisible for long periods of time - once even for three days. She enjoyed doing this because it meant she could hang out with her mom without her saying "mean things" to her.
Lili Zanotto, if you squint real hard. There's the subtle ways she reacts to Raz's speech and his talk with Dogen- ("Back home your powers make you an outcast. A loner." and "Punished... for having powers I never asked for.") the fact that she calls friendship bracelets stupid* "Why? So you can unbutton it and take it off when you got tired of being friends with me?", but still wears one, how jaded she is about the Psychonauts, and the fact that Raz and Dogen are the only two campers she talks to...