Large story, lots of NPCs with stuff to say, Massive amount of enemies (that all needed to be modeled) with different powers that can be taken, and used. All those abilities would need to be programmed, lots of enviroments, advanced character AI for the beasty, so it will not only be able to navigate the enviroments with the player, but also fight well, too. The game would take too much work, because it is a lot more complicated than a generic platformer. The game would be a platformer/RPG hybrid, and that RPG part takes lots of work. The EXP system would be a bit complicated, too. And that is just the hardest stuff.

Simple, huh?