I pick CON side, so opponent has to argue PRO side. He can call me "pro" or CON, to avoid confusion, the all caps is the side we are actually on.
I will merely open up with common logic ideas. A world government would be incredibly difficult to sustain. How would it even work? Can it enforce laws? The different ideologies among countries and vast bureaucracy would make enforcing ideas near impossible, and become about the individual countries anyways.
Is the world government even necessary? US is already considering itself the "police of freedom", exercising its control over Iraq war and terrorists who would oppose the World Government. They hold the majority of military power, technology and force, making it unnecessary to distribute resources to other countries to enforce this peace, or even assist in areas that need help.
How can the world government prevent corruption? Having such responsibility and power is difficult to place in the hands of a few people alone. How would these people be decided? How would they be able to enforce specific laws for the world in general?
Why the UN? There are other organization that may be able to establish the same goal, such as EU, WTO, and other ideas that may be better organized. Because the UN's leaders come from five drastically different countries, the vetoes mean that they can hardly accomplish any laws at all. The EU at least, is similar in concepts (European ideology) such that they may potentially be able to establish the world government better than the UN.
I await con's answers.