Complete Monster: The Queen has been emotionally abusing her own daughter, Yorda, for her entire life, imprisoning her in a small cage hanging from a ceiling as well as manipulating her into thinking that she'll die if she ever leaves the castle, effectively breaking her spirit. It's implied that this is the reason why Yorda is so passive during the game; it's not because she's weak, it's because she's broken. She's also been manipulating a village into sacrificing their children, with said sacrifice involving sealing those children inside of coffins until they die; it's also implied that she controls their spirits after death as her shadow-monster servants.
Ico's character design can inspire this. If you look closely at where his bandages meet his horns, it looks like they've been torn, as if the horns sprouted abruptly. He also seems to be wearing normal children's clothes. He must've been taken away suddenly, without giving him, his family, and his friends any time to cope emotionally.
There are numerous graves throughout the castle, seeming to have been built all at the same time. They're just curiosities until you remember that they're all meant for the Queen. Just how long has she planned for her immortality and how long has she been alive?
Game-Breaker: The lightsaber will clearly save you a lot of trouble, considering how tedious the shadows can be to fight with a normal weapon.
Genre Turning Point: ICO is credited with ushering in the arthouse game, and for its mammoth influence on visual and environmental storytelling over cutscenes and dialogues. The mix of puzzles, platforming, and combat in both games was a huge influence on the revived Sands of Time trilogy of Prince of Persia and the Uncharted games, as well as codifying the importance of a supporting NPC companion, inspiring the likes of Alyx Vance (Half-Life 2), Elizabeth (Bioshock Infinite), and Ellie (The Last of Us). Its cinematic presentation, heavy emphasis on art direction and visuals for atmosphere and setting has likewise been cited as a major influence on FromSoftware's Dark Souls and Bloodborne.
Goddamned Bats: All the shadows are this. They won't do any significant damage (except if they push you off a ledge), but will get in your way, and you spend much time waving your stick in the air while trying to hit them.
Good Bad Bugs: In the original PAL version, setting the framerate to a certain setting allowed you to jump amazingly high with carefully-timed button presses. This made it possible to finish the game in less than an hour.
Getting the secret weapons requires you to throw a ball into a large container. You could spend 1 minute to 1 hour trying to do this. The other way is to put the ball on the spot where the container rises from, then activate the floor switch.
Finding the secret weapons. If you are not informed (or don't look at the trophies in the PS3 edition), you won't even suspect there are secret weapons! After all, weapons don't really have much importance to begin with and ICO isn't the kind of game where you would expect such Easter eggs.
When you enter the East Arena for the first time, it will seem like you've hit a dead end. It turns out that the door to the next room is hidden in the shadows. It doesn't matter how high you turn up the brightness in-game or on your television screen. The corner of the room with the door is pitch-black. You can be stuck for hours looking for hidden switches and whacking everything with your stick.
Iron Woobie: Ico has been through a lot. He's been taken away from his family to be sacrificed for God knows what reason, he is trapped in a castle without knowing a way out, and he probably hasn't eaten or drank in a while. However, he is still strong-willed enough to find a way out of the castle, fight monsters, and help a girl—whom he doesn't even know—from getting her body taken over by her powerful and evil mother even if he has to fight her.
Moral Event Horizon: At first, the Queen seems to be a worried, if arrogant mother who just wants her daughter back. But once it's revealed that she plans to kill Yorda and use her body to extend her own life...
Player Punch: Returning to the hall of pods where Ico started the game from after Ico is separated from Yorda — only to find her surrounded by shadows of horned boys like himself and turned to stone. Then, killing all those shadows who mean Ico no harm at all.
Some players think that the Escort Mission aspect of the game is no better implemented and just as annoying as in any other title, significantly damaging a potentially quite effective platforming game.
The camera system can be outright nauseating and frustrating for players, and most of the time it doesn't really help you, either. It also makes looking at something distant a total pain, since you can't hold the camera steady with a joystick, and releasing the joystick will only cause it to jerk back towards its predetermined angle. At times, it also doesn't account for obstacles blocking you from the camera angle's line of sight.
That One Achievement: There's a trophy in the PS3 version that requires clearing the game in under 2 hours. It can be done with practice, but the annoying part comes from the fact that the timer does not stop until the events on the beach after the credits and the timer continues to run during the credits. Meaning you have to beat the final boss in about 1:45-1:50 in order to make it.