It's really no secret... there are 2 reasons
1. World Class Developers who really know what they're doing and aren't afraid to explore the benifits of new hardware.
and
2. A game designed from the ground up with the Xbox 360 in mind, and as much time devoted to it as possible.
think about #2 for a minute... what other 360 game is exclusive to that platform and always been exclusive to that platform from day 1?
There arn't many, and the few that are are beautiful... PGR3, DOA4, and the Mystwalker/Q entertainemnt stuff (which honestly we haven't seen much of).