In Ico you have to find places to climb, push around crates and boxes, pull levels, climb and swing...this is what I mean by poking around and explorin, and also the process of proceeding from room to room.
This as opposed to games where the environment is basically something you run through as you shoot things without feeling around and solving puzzles and then at the end you might get graded or maybe you level up and buy weapons.
I think ICO took a lot of ideas I associate with character action-adventure games, specifically Nintendo's, and put them into a different context, a style directly inspired by modernist movements in art, surrealism, and yes stripped things down to an essence of what that art implies rather than focusing on "get the magic flaming arrows at the other end of an huge map, and guess what no clues, so see ya in a few hours", focusing on the subtle nuances of two characters and their interacting as well. Which is a big differance, yes.