Romney's administration, when in power, would need to moderate a lot of the rhetoric designed to get the base enraged at Obama's "weakness". I do think US policy might be a bit more abrasive and assertive, but realistically there's not many places where Romney would act differently from Obama. China policy would likely remain the same, as would North Korea policy. European policy would not likely change, not because Republicans haven't been hawkish on expanding NATO, but because the two candidates for expansion (Ukraine and Georgia) won't get in anyway (Ukraine shifted to the pro-Russian Yanukovich, and other NATO members are certain to veto Georgia based on 2008). The missile shield pipe dream might cause some friction, but that's about it.

In the Middle East, however, I think that Romney would authorize military action against Iran. The Syrian regime would fall faster, Iraq would probably enter total civil war and Israel would take advantage of the chaos to hit Hezbollah - in other words, the war would become a Sunni-Shia bloodbath.