One of my favorite games of all-time, and definitely my favorite Double Fine game. Psychonauts is a bundle of joy and love.
There is nothing I dislike about this game, at all. I used to play a lot of funny platforming game years and years ago, but don't as much now. I loved playing things like Crash Bandicoot or, going back further generations, to things like Earthworm Jim. When it comes to weird humor though, Psychonauts is probably the top.
The story of Psychonauts starts out simple but gets significantly more involved as it goes along, getting decently complex as things actually become a mystery. But the way of solving the puzzles is the best. Delving into the minds of different characters along the way, using their thoughts as levels, is awesome. Each stage in the game is so unique and interesting that I will replay them all forever. All of the characters are so interesting and have their own ticks, and the graphics and sounds (including voice acting) are so well-done. Nothing is wrong at all with the design.
Raz, your character, gets interesting power-ups along the way, making exploration fun and opening up random puzzles. Finding the easter eggs in the levels is fun, from the basic collectibles (enabling you to buy upgrades) to especially the memories. The most stand-out ones for me are in Milla's mind, but just about all of them are rewarding in some way.
The platforming itself is clean and not too frustrating, though some levels take a bit of adjusting to, such as the angles in the Milkman stage. And exploring outside of the levels is just as fun because Double Fine developed the world with lots of detail. Wandering around and finding some little incidental cutscene is great, especially since nearly everything in the game is really funny and off-the-wall.
If you don't find the game's sense of humor enjoyable, then that's a shame. But I have to say that this game is one of the most enjoyable experiences I have ever had. Go play it, now!