The power of the desktop PC with the power consumption of today's smartphones. Nothing Apple or Android on ARM will be able to compete with that. Microsoft was late overall to the modern smartphone game, but the will win the end due to superior technology.
um... im pretty sure by the time wp9 comes out it'll be something else (whatever comes after medfield), but as far as i know medfield is a great first step in the right direction, but still has to improve alot before it's competitive with current Qualcomm processors
(again this is all just from reading news.. no hands on)