Well, for me:
1 - market position of the Wii. This is obvious. Right now the Wii is the fix of choice for motion control and has gobbled up a huge percent of the market presumably interested in motion controls for games such as MS has shown - how much is left, for Natal or Sony?
2 - Natal looks to me like it will need some serious space for some titles. I guess you could argue the same for Wii and Sony, but a physical wand means you can stand still while just moving one arm, Natal, as 'you are the controller' needs you to make all inputs via your body. So either the inputs will be very minimal for some games, or you're going to need a lot of room (and energy) to constantly move your arms, legs, etc. vs using a Nunchuck and Mote. I'm not sure how practical Natal is really going to prove for many households.
3 - while Natal looks great in concept, I'm still of the view it's actually the weakest motion control option for games (although the best as an interface). The main issue for me is the lack of feedback due to lack of controller. Essentially, you're mugging in air with no feedback. I think that in general, the average person, despite all the 'controller as barrier' messages from MS would prefer feedback. Take Tennis. With Natal, as advertised, I simply swing my arm, no feedback, no rumble, nothing, just a visual response on my TV. With Wii Mote (or Sony Wand I presume) I swing my arm, get appropriate feedback via rumble I've 'struck' the ball and get the same visual cue. Therefore both the Nintendo and Sony options will feel more natural and immersive. Our bodies know its not natural (or immersive) to get no physical feedback. It's like when Sony tried to diss rumble due to their lawsuit - everyone knew it was nonsense, and the second you played the same game with rumble vs no feedback it just felt superior.
The big advantage Natal could have, in my view, is if MS get good traction with it as an input device for PC and Natal first, getting people used to it in concept then building out with the games as they come up with titles that work well with it.
Try to be reasonable... its easier than you think...
