During a Q&A at E3, Nintendo's Shigeru Miyamoto was asked what he thought about Sony and Microsoft's motion controllers. He brushed the question off, mentioning how it isn't Nintendo's policy to show off technology without first implementing it into games [He must have missed his company's presentation of the Wii Vitality Sensor – Ed.]. The point he was indirectly trying to make, however, was that there weren't a lot of solid games to back up Sony and Microsoft's new motion sensing technology yet, and the Wii has plenty.
Nintendo has spent the past few years building up titles like Wii Sports, Wii Fit, Wii Play and Wii Sports Resort that can be picked up and played by almost anyone and have a sort of inviting charm to them. Sony and Microsoft obviously aren't getting those titles for their systems, so they'll have to try and create them from scratch. Furthermore, third parties will have to adjust to the new controls, especially Natal, which is even more different than the Wii than Sony's tech. Look at how long it's taken publishers to adapt and figure out Wii. The learning curve starts all over again.
And on the first-party side, besides the obvious Wii titles mentioned above, Nintendo also has a lineup of titles that manage to cross traditional lines of "core" and "casual" with IP appealing to a wide variety of demographics. Games like Mario Kart Wii, Super Smash Bros. Brawl, Super Mario Galaxy and The Legend of Zelda: Twilight Princess are all hit titles exclusive to Nintendo, and Sony and Microsoft will have difficulty replicating their level of success using their motion controllers.