What would the world be like if there was only a single government? The world would be very different if the hundreds of nations of the world today were united under one government. There would be some advantages and some disadvantages to having a world government. Although there are some good aspects of a global nation, I don't believe that this type of system would work in the world today.
If the world consisted of one solitary government we would hypothetically have reached a point of world peace. With one army and navy there would be less fighting and conflicts between nations. If conflict did break out it would be considered a "Civil War" among the one nation of the world. There would also be advantages in the trade industry. Without the taxes and tariffs the trade industry deals with today, importing and exporting goods would become much easier. We would have one court system, and the governmental powers would be shared among the worlds nations. Having one government made and operated by the entire world rather than hundreds of governments around the world would in some ways be helpful.
A solitary government would also have many disadvantages. Having one set of laws and policies would cause conflict between different social groups. If you didn't agree with the world constitution you would not have the other options you have today. When one doesn't agree with the laws placed before them, there other options with many governments yet no options with a single government. Also, with so many leaders in the world, it would be hard to keep the government from becoming a dictatorship. If too much control was given to one person, it could turn into a very serious situation. If you were accused of a crime, there would be no way to escape punishment. Today if you are accused of a crime you can escape to a different country. This idea may cut back on the crime scene, but it would also force people who don't agree with the laws to obey them or get punished for their actions.