Psychonauts instantly hooked me after starting up a new game. It plays as a standard platformer, like Super Mario 64 or Jak & Daxter, but with a unique cast of characters and top notch writing, elevates itself above similar titles and ends up appealing to a wider range of tastes as a result.
The gameplay is good, but not great. The platforming is a little slower paced than similar titles, but level designs are incredible, and helped by equally incredible visuals. Most levels take place inside characters' minds, so each of these is unique to the "host, " and makes for a new experience in each section.
Characterization is where Psychonauts truly shines. Abandoning normality, the game's cast is made up of psychic camp-goers and their counselors, monstrous fish, insane milkmen, and mad scientists, to name a few. Writing and character stories are a huge focus in this game, and a slightly dark sense of humor emphasizes top-notch writing and voice acting.
Psychonauts impressed me not because it's the best game ever, it isn't, but because it's more entertaining as a sort of "cinematic" experience with some above average gameplay thrown in.