Accidental Aesop: There's always hope no matter how hopeless a mental affliction may seem. Supportive assistance can help someone heal the source of their problems, and even realize they they don't need Take Over the World to feel validated.
Is the Hand of Galochio real, or is it Raz's own psychic powers unconsciously manifesting his fears brought on by his family's superstition?
Since the only real image we see of the Butcher 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.
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.
Breather Level: The Milkman Conspiracy level is much less action-oriented than the other levels of the game, with the focus being on Raz finding different objects in order to progress to certain areas. There are no enemies besides the two Nightmares and the Den Mother Boss Battles and the platforming, while present, is actually fairly tame.
Demonic Spiders: The Tragedy variations of the stage actors are some of the trickiest monsters to fight in the game, mainly due to how all three variations appear at once from the first time one changes the mood lighting of the stage.
The Flower children spin like saws along, and are effectively invincible for that time until they briefly stop.
Hell hounds breath fire at you (which of course makes Raz run around almost uncontrollably when lit) and can turn to face Raz while they attack.
Harpies fire shots at you while suspended in the air, and can push you back with a screech if you manage to get close
The exploding rats in the upper floors of the Twisted Towers, while easy to kill, come at you frequently in big enough numbers that you might not be able to shoot them all from a distance in time, can sneak up behind you if you're not looking, and take off a chunk of your health if you get caught in their explosion leaving you confused and vulnerable to more exploding rats.
Fangirls loooove Sasha Nein. There is more fanart of him than any other character in the game.
Alternatively, D'artagan (sic)—a character cut from the final game, who nonetheless appears in lots of fanart, occasionally shipped with the hero Raz.
If you look you can find a following for just about every student in the camp.
Of the other adults, Ford Cruller especially gets a lot of the fans' love. His Cloud Cuckoo Lander attitude and tragic backstory make him adorable. Being a old man who's obsessed with bacon makes him hilarious. He's got something for everyone.
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.
Fanon: Loboto being Bobby's father, due to the similarities in their appearance and personality.
Genius Bonus: The Waterloo World level's background music is composed of the lesser known portions of the 1812 Overture.
Goddamn Bats: The exploding rats in the asylum are quite possibly the most annoying creatures in the game.
Harsher in Hindsight: Loboto's "if the tooth is bad, we pull it" comment is a bit less amusing due to The Reveal in Rhombus of Ruin namely, that his psychic-hating parents had him get a lobotomy to get rid of his powers.
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: Warrior Princess, 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.
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.
Still, as of 2016, the Steam Mac version is the inferior one.
Meat Circus, infamous for its difficulty. It combines Timed Missions, Escort Missions, Rise to the Challenge, a Timed Jumping puzzle where the boss is trying to knock you off with attacks from offscreen, and virtually the same boss fight three times. Five years after the game's release on Steam, a patch was made available that made the Meat Circus easier. The last fight totally makes up for the rest of the stage, though.
The upper levels of the asylum. They're dark, have some tricky jumps, and are home to some really annoying enemies.
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.
Gloria's Theater can be a pain to get through because of all the intentionally awful acting which plays for each set and mood, the path to progress being rather obtuse on the first playthrough, and just generally not being as entertaining or creative as the levels that come before or after. The second part also features some egregious video game sins like tricky platforming on tight catwalks with lots of enemies, and, worst of all, Camera Screw.
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 anything 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.
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...
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...