[quote name='Sideswiper']I don't understand why so many people like this game. From what I played of the demo, it's a jumping game.[/QUOTE]
It has the most all around engrossing, engaging, imaginative, and plain fun story of just about any game out there. That's why it's so popular.
Personally, I've probably played all the way through the single player of maybe 20-30 games in the last ten years (after purchasing or renting 300-400 (rough estimate, over 200 games on Steam alone).) I played through Psychonauts 3 times, and probably will again in the future.
I've been waiting a long time for this to drop just so I could buy it again on Steam. Great deal!