Forestman said:
We need a one world government too:
Stop man made climate change.
Stop terrorism.
Stop over population.
Stop the threat of nuclear war.
Create a stable economy.
Create a defense against asteroids and solar flares.
I think it would be better to pick at the individual statements than to criticize the statement as a whole. Most of these ideas can be thought out logically using historical examples without picking at the OP himself.
Stop man made climate change -
This certainly could be achieved by a world-wide regulating authority assuming that we have enough of an impact on climate to make a change by regulating industry. That would be another topic entirely.
Stop Terrorism -
This is impossible. According to the US Department of Defense's http://terrorism.about.com/od/whatisterroris1/ss/DefineTerrorism_4.htm"
definition of terrorism.
The calculated use of unlawful violence or threat of unlawful violence to inculcate fear; intended to coerce or to intimidate governments or societies in the pursuit of goals that are generally political, religious, or ideological.
By this definition you can never really stop terrorism, any more than you can stop every individual in the world from committing unlawful acts.
The only affect a world government would have is preventing a state sponsored terrorism. This would be a mute point because you would only be stopping state sponsored terrorism because their would only be one state. On another note, what stops the single world government from committing terrorism.
Stop Overpopulation -
A touchy subject in itself, this could theoretically be solved by having a single world government by implementing laws regulating the number of children people can have. I can just imagine how we would have alternating periods where everyone is required to have one child, followed by maintenance periods where we are required to have two children to maintain optimal population followed by periods where we have to have three in order to increase the population during periods that are dwindling. Like the climate change argument, this could be discussed in its own thread.
Stop the Threat of Nuclear War-
Mutually assured destruction is just as strong a deterrent as a hypothetical world government. A world government could theoretically dismantle all nuclear weapons, but to what end. We live in the post-nuclear age, we are going to have to accept that these weapons exist and come to a balance. If we dismantle them all, as time progresses the possibility always arises that through need or evil intent they could show up again. The other problem I can see is that with a single world government, there would be no way to hold your government accountable should it opt to use nuclear weapons in an isolated incident. It seems to me that there will always be a threat of war and of great loss of life. Should a world war arise between large groups of people, these extremely efficient weapons will rear their ugly head again and again.
Create a Stable Economy-
Unifying currencies has not shown, so far, any improvement in stabilizing economies. When you unify currencies you tie the worth of your money to the stability of every local and regional economy in the currencies reach. The up-side is that should, through political or agricultural turmoil, a local economy is doing poorly the currency doesn't lose worth and inflation is less of an issue. The reason for this is that the stronger economies can prop up the currency until the local area stabilizes. The downside is that the more widespread economic decline the larger the burden on the other countries and a wide-spread failure is more likely to affect a larger area.
For instance a decline in one country can stifle the growth of another country. Stifling growth is roughly equivalent to initiating decline economically. The results in both countries ending up in a decline and this snowballs into a general decline.
It just doesn't make economic sense to put all your eggs in one basket. This isn't idealistic, but there is no such thing as an perfect economic system.
Create a defense against asteroids and solar flairs -
Sadly I can't really come up with a discussion to address this. The subject matter is a little far fetched. This is roughly like saying we need a defense against the Earth ending. Eventually technology will reach a point where defending against these problems will be feasible. A unified government certainly could coordinate to find a defense, but at what cost economically, and socially. At best, it would speed up a defense, but not really so much so that it would be worth the drawbacks in other areas. By the time we developed the tech to destroy, move entire asteroids we may also have the means to colonize other planets and evacuate Earth. There are just too many variables.
Other issues to consider -
Politics -
How much more frustrating will politics become with world government. Think about what it is where your from multiplied by the number of people who don't live in your country. You have to accept that the most popular (not most able) people get elected and that politics are inherently corrupt. You accept these facts like any reasonable person accepts the duality of man.
Bureaucracy -
The operation of government would be so complex that it would be near impossible to manage it, and near absolutely impossible to run efficiently.
(I did read the posts between the first and this, but I didn't really feel like drawing on those arguments just yet.)