Thers only 2 things for me - Price and games (that support NATAL).
1. Price - Simply put, I wouldn't pay more then $70 (the price of a game) for NATAL. Simple as that. I'm not paying $71.
2. I need there to be good games for it. I'm not buying NATAL so I can play a bunch of lousy minigames like "kick the soccerball" or "lame dancing game 3". I would want lots of cool games like sports games where it picks up your motion or a boxing game would be fun where you have to punch and dodge in real motion.