Difficulties

Limited additional benefit with extra cost
Some economists argued that a single world currency was unnecessary, because the U.S. dollar was providing many of the benefits of a world currency while avoiding some of the costs.[15] If the world does not form an optimum currency area, then it would be economically inefficient for the world to share one currency.
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Economically incompatible nations
In the present world, nations are not able to work together closely enough to be able to produce and support a common currency. There has to be a high level of trust between different countries before a true world currency could be created. A world currency might even undermine national sovereignty of smaller states.
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Wealth redistribution

The interest rate set by the central bank indirectly determines the interest rate customers must pay on their bank loans. This interest rate affects the rate of interest among individuals, investments, and countries. Lending to the poor involves more risk than lending to the rich. As a result of the larger differences in wealth in different areas of the world, a central bank's ability to set interest rate to make the area prosper will be increasingly compromised, since it places wealthiest regions in conflict with the poorest regions in debt.