Democracy
The key to ANY government, world or otherwise, is NOT to have a pure "democracy." The Founding Fathers in America realized that there are some things that should not be subject to majority rule, or the state with the largest population, and set up the government accordingly. One assumes we could do the same thing internationally, but I'm not that impressed with how our government's original committment to limited authority has turned out, and I suspect a world government might follow the same path.