No one can.
I watched the videos on XBox.com for the first time in six months yesterday again. It re-opened my eyes and I can see a lot of potential, but I can also see the possibility of failure.
No one will know until after it ships. But, one thing is certain, MS seems to be coming to the poker table with a lot of chips to ante on this one.
Originally, I felt it was only a casual thing with a probably focus on physical games and fitness (Karate, Aerobics, etc.). But, as I have seen some of the demos, I see a lot of potential for hard cord games like RPGs, RTS, etc. The biggest problem a living room console has is the lack of keyboard and mouse... Natal has the potential to eliminate that problem. Voice recognition and full body motion detection may make the keyboard and mouse irrelevant going forward. And if MS does what they appear to be doing and put Natal on PCs as well, we could be looking at further communication and collaboration innovations in the workplace as well with Natal.
The more I see, the more I realize the goal is much grander for Natal then just gaming. It could be the new interface between humans and computers/consoles. People may look back in five years and think keyboards and mice are useless add-on devices, just like punch cards and assembly went extinct in the early 90's.