I think that the fact that Natal is coming out several years after the 360's introduction will help. The Wiimote/accelerometer-based-control has been marketed as an integral part (probably the most recognizable part) of the Wii experience. In keeping with that, most people don't own Gamecube-style controllers, so making games that use a more traditional interface is a losing proposition on the Wii.
In contrast, Natal is a purely optional add-on, and I'm not sure on this but I'm not sure it will be possible to control non-Natal-enabled games or the Xbox 360 dashboard without a traditional controller. This way, game developers can choose the controller paradigm that makes sense for the game, and the Natal ecosystem can grow organically, rather than have it foisted upon people. Maybe this is a recipe for languishing and fragmentation, but I can see how this strategy could work out better for Microsoft than Nintendo's did for them.
The technology could also be intrinsically much better, who knows?
Optional peripherals are a chicken-and-egg problem. Consumers won't buy it if there are no compelling games for it, and publishers aren't willing to commit to the peripheral (e.g., controllers not allowed) if there are not enough consumers that own the peripheral.
Microsoft can address this by stepping up and making Natal a required accessory for some hit game. The trick then is finding Natal's "killer app", which we have yet to really see.
It's going to be tough - see Microsoft's FF Steering wheel and the 360 webcam as evidence. The wheel is confined to racing games and isn't useful for much more, and the webcam is fun for a few nights but doesn't have a lasting appeal. The only webcam game I'm aware of was the one shipped by MS, and it's not very fun (and very tiring on the arms). Both devices are successful in their own scenarios, but are still niche products (how many games have you seen that use the camera?).
In contrast, Natal is a purely optional add-on, and I'm not sure on this but I'm not sure it will be possible to control non-Natal-enabled games or the Xbox 360 dashboard without a traditional controller. This way, game developers can choose the controller paradigm that makes sense for the game, and the Natal ecosystem can grow organically, rather than have it foisted upon people. Maybe this is a recipe for languishing and fragmentation, but I can see how this strategy could work out better for Microsoft than Nintendo's did for them.
The technology could also be intrinsically much better, who knows?