Ico is one of those few games that are true sleepers. It is a game that adheres to "less is more" design and cumulates to a spectacular game that challenges everything you though could be done with a hybrid platform/puzzle game.
What makes this game so legendary is that there is nothing to the game and yet there it is full of things to do and see. There are only three characters in the entire game yet these three gives the entire game life. There are no complex moves yet you can climb and negotiate and move through the game. There is no life bar but that doesn't make the environment any less hostile and multiple ways to die or fail. There is very little music and sound effects are minimal which lends itself to the stark and isolated nature of the environment.
The story is simple too: You are a boy named Ico who has been cursed with horns growing out of your head. The village sends you to a decaying and mysterious castle at the edge of ocean like they have done with all of the others who also have horns and leave you to whatever happens to those abandoned inside of the castle. Having no intentions of dying here you escape your confines and find another prisoner, a ghostly white girl. Together you must open the gate and make your escape before both of you are consumed by whatever controls the castle.
One of the best features of Ico is that not only you must survive you must help and protect the girl you found as well. This interaction makes for a much more complex level of play than previous games before it. It might be easy for you to negotiate a room or a wall with just your able bodied self but the girl can't climb as high or jump as far as you so part of the challenge is to open up opportunities to lead her safely around the game. The girl interacts with the environment on her own (they devoted programmers just for her behavior and actions). She will go towards things she finds interesting or wants to play with (watch how she interacts with birds) and will run away from things that scare her (watch how she jumps when you drop something heavy near her). As it turns out the "monsters" in Ico are more interested in capturing her than you so they will chase her if you aren't nearby to fend them off. Although her actions make her seem weak and a burden she does serve one important purpose: she can open the special doors scattered around the castle. Even though you are "the muscle" to the partnership both of you must make your way out of the castle.
The other grand feature of Ico is the castle setting itself. The entire game takes place inside the castle where the entire point is to get out and there is a lot of moving around that needs to be done. Not only is the environment large but it is very complete. There are no optional "secret areas" or side quests to the game. You must solve all of the puzzles to complete the game which means upon completion you have seen *everything* in the game.
Ico is one of those special games that is so enduring and complete yet lacked high marketing or exposure. Simply put there hasn't been a game quite like it before or after.