I've got to agree with HappySqrl, the 360 lineup is lacking certain genres. The only noteworthy platformer on the system is Banjo Kazooie which is still at least a year away. Adventure games? Maybe Kameo...and that's about it. It's also severely lacking in Japanese games (that aren't exclusive JRPGs paid for by Microsoft) and cute E-rated games. Microsoft has the opposite problem of Nintendo - it's not "kiddie" enough.
Edit: Not that this prevents MS from doing well, but I think it will prevent them from being #1 unless they can find new audiences.