Romney would be an utter disaster for the USA, and by extension, the world. Mainly because if Romney snags the White House, it will be by riding a wave of fear and anger that will almost certainly gain the Republicans the Senate as well, and that would give us a Federal government in which the GOP controls all three branches.
Romney would then proceed to rubber stamp the Republican agenda like a good little robot, and massive amounts of the tax burden will be shifted off of the wealthy and corporations and on to the shoulders of the poor and middle class. (It is an inescapable fact that Romney's "economic" plans would hand major tax cuts to the wealthy while actually increasing the tax burden on the middle and working classes.)
And with that you have what is likely the last nail in the coffin of the American middle class, and when we go, the entire US economy goes with us. The fact that Romney and his GOP puppeteers either do not understand this fact or do not care is utterly terrifying. If they don't understand, then they are willfully, deliberately, astonishingly stupid. If they don't care, then they are clearly evil in the classical sense of the word. Neither option is comforting.