Because some of us don't want gaming to turn into some sort of fitness competition. I'd much rather sit on my behind drinking a beer whilst playing games.
I've been shockingly bad in researching natal, but is it not likely that the classic controller interface will remain as strong as ever, but with natal being an enhancement for -some- games?.. I'm completely prepared to be shown wrong in this but it seems that this overall negative reaction assumes the intention to migrate almost all control functions of 360 games to movement control in natal.. this seems..simply..unfeasable as it stands, but again, please do correct me
