What people don't understand is that Nintendo reached the casual gamer because of the core gamer. It was core nintendo fans, the ones who grew up with metroid, mario, zelda, etc., who showed their own family members and friends wii sports and wii play. It was core nintendo fans who convinced their family members to get wii fit. And this worked because believe it or not, core gamers do play those games. Wii music failed because core gamers did not get behind it and spread it to casual gamers. I gurantee you if more core gamers bought wii music, it would have taken off with the casual gamer because the core gamer drives the casual gamer.
But Natal is completely eliminating the controller. If the core gamer isn't there to act as an ambassador to spread the enthusiasm to the casual gamer, it will fail.
Sony on the other hand is doing everything nintendo does, but with higher tech. Sony is a bigger threat. Core gamers will get behind the wand but not behind natal. And core gamers are what drive the casual gamer. The casual gamer does not read gamepro and stop at gamestop every month. They get their games because of the core gamers who put peer pressure on them.