The Metro design principles center on a look that uses type to echo the visual language of airport and metro system signage. The goal is to clearly direct end users to the context they want.

METRO IS OUR CODE
NAME FOR OUR
DESIGN LANGUAGE.
WE CALL IT METRO
BECAUSE IT’S MODERN
AND CLEAN. IT’S FAST
AND IN MOTION.
IT’S ABOUT CONTENT
AND TYPOGRAPHY.
AND IT’S ENTIRELY
AUTHENTIC.
-- MSFT Windows Phone design principles