OK, so a real full one world government might be too much to ask for but, at the same time, what is wrong with the idea exactly?
1.
It is unnecessary- International law, treaties, agreements and alliances already exist, and have existed for hundreds, if not thousands, of years.
2.
It demands a homogeneous world- what form of government are you proposing, exactly? Assuming a Democratic Republic, how exactly do you intend to get the Chinese to go for it? Assuming something other than a Democratic Republic, how do you intend to sell this to the western world?
3.
Economic inequality must be addressed first- the economic disparity between the first world and third world is enormous. Any world government would be required to tax the developed world to fund services in the third world. Another example: it would be impossible (at present) to govern the entire world with a single "minimum wage" law; it would demand upper-class salaries in the developing world, thus hindering economic development.
4.
What about war? Assuming all military was controlled by the same government, the very concept of military is obsolete. Any war would be either a civil war, or the act of terrorist minorities, rendering advanced weaponry needlessly dangerous (at least until an alien invasion). At the same time, total disarmament is equally impossible; crime and law enforcement will both rely on force, and some form of weaponry will survive.
5.
What happens when we elect a jerkface? Even in the "best" democracies, occasionally a lousy President is elected (I'm not naming names). Some of the most ferocious fascists in history were elected; democracy is no guarantee of good leadership. At present, even the worst despot is kept in check by other nations. If we remove this balance, what happens when the next Adolph Hitler is elected? Fascist state, or global civil war? Either way, it doesn't sound like much fun to me.
Just one dude's opinion.