North Korea ?
China has more to worry about from North Korea than us. I'm thinking either China itself or regional war in the Middle East. The Arab Spring, if it can be sustained, reduces the chances of a major war in that region of the world.
Not necessarily, it all depends on the governments that rise up to take the place of the ones that fall. New Islamic governments could rise up in Syria, Libya and Yemen and that is NOT good for us at all.
Always a possibility, but those fighting for freedom from their current governments are unlikely to trade a secular dictatorship for a religious dictatorship.