I think Natal's strength will be in unconventional implementations where the idea is to gain immersion by 'hiding' the interface from the user. I'm thinking something like Heavy Rain could be engineered to work by natural motions, and have the player experience it with instinctive movements. eg. To fight, the player woudl natural take a swing at their assailant, which would be interpreted as the attack option. The choice of fast and slow motions should come without thinking, in contrast to deft thumb control on a traditional stick.
I'm sure it'd take a lot of prototyping to get an ideal system, and perhaps we won't really get anything of that ilk before the next round of hardware, but Natal should start it this gen I hope, and get the ball rolling for some futuristic experiences.