You say the platforming, but the platforming is a core of only a few of the levels. The Milkman Conspiracy, Black Velvet, Lungfishopolis, and the battle of Waterloo are all very adventure-y. Black Velvet level is full of great platforming too in that you have to stay out of the way of the bull's path and use sewers and signage with speed.
The other levels, such as Mila and the coach's levels are fucking fun and Sasha's level has barely any platforming in it and is primarily an action stage.
Psychonauts has very inventive and creative level design. You can bring up the platforming all you want, but Psychonauts is in no way a pure platformer, and I even find the label of platformer to be entirely wrong due to the myriad of gameplay styles the game has. The only really bad level is the Meat Circus.
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