A lot of core games are "hard" or "complicated" because the interaction with the game's interface is difficult. The perfect example of this are the classic Resident Evil games where shooting a zombie was a challenge because you had no way to (really) line up a shot. With a conventional controller you're limited in the level of interactivity you can have in a game by the number of buttons you have, because adding additional functionality beyond that point increases complexity; and eventually the game becomes unaccessable to many people.
Really expressive motion controlls can answer these problems because there is the opportunity to more closely match the interaction with the game's environment than could ever be done using a button; and there is far more functionality that can be added to a game because there are far more potential motions that can be used than there are buttons on a controller.
The challenge (of course) is adding these motion controlls so that they're a meaningful enhancement to the game, and not just adding motion controlls for the sake of adding motion controlls.
