I don't see any good reasons to believe that global governance is necessary or desirable. On the other hand, the only reason I can see it being worse than currently existing governments is that would have much more power.
My view of states is fairly cynical. While there may be historical reasons why Belgium, for example, is a more valid nation-state than SouthDutchandFrenchistan, that doesn't invalidate my view that all government, unless you live in a city-state, is a bunch of people living far away from you, deciding to do lots of things and then saddling you with a bill. My personal opinion, rootless cosmopolitan that I am, is that being ruled from London is no different than being ruled from Brussels (or Washington) in the grand scheme of things. As such, being ruled by the United Federation of Planets, out of San Francisco (or, more likely, Geneva) wouldn't make a huge change in my opinion. It would be a bunch of people, far away, delivering goods and services, some of questionable utility, others fairly useful, and charging me for them all. Meh.
I would also posit that there is no reason to believe global government would bring about the end of war. China was One Empire Under Heaven, yet was plagued by all kinds of civil wars, secessionist movements headed by rogue warlords, peasant uprisings, fanatical cults and insurgencies. Europe was the same - in theory, it was Christendom, under the rule of the Pope and the Holy Roman Emperor, but the Byzantines and French tested that proposition and came away with rather different conclusions. The Pope's blessing was required to be seen as a legitimate ruler, and often the Pope would revoke that blessing - going as far as to excommunicate entire country's of people. And of course, there were the wars of dynastic succession and so on, battling for relative power and prestige within the confines of the existing system.
The other major issues would be that of culture and religion, and on what grounds would world government procede? A lot of assumptions are that world government would be necessarily western in outlook - liberal, respectful of human rights, involve devolved power and multiple branches of government, secular.
But Westerners only make up a fraction of the world population. A democratic world government would have to accomodate the wishes of a billion Muslims, many of whom are insistent on trying out this Shariah Law thing they keep reading about. It would have to contend with 1.3 billion Chinese - who even if they were not under the rule of the Communist Party, would still be heavily informed by a strong history of Confucian legalism. And then there is nearly a billion Indians - many of whom are, of course Hindu. Not forgetting the nearly two billion Christians - many from places like Uganda. Already we're looking at a world where being homosexual would be very dangerous.
Without the withering away of these religious impulses and some kind of convergence on Western social and ethical norms (which, unlike
certain people, I do not consider inevitable or irreversible), you can either have democratic global government, or you can have liberal global government, but you cannot have both.
As for regional bodies...they're mostly range between "ineffectual but harmless" (ASEAN) and "somewhat useful" (ECOWAS). The main problem is that a lot of them seem to look to the EU for guidance, and the EU is, to put it mildly, not good at the whole democracy thing. Still, regionalism as a general approach makes sense. It allows for easier coordination of international issues, disease, smuggling, terrorism etc, usually they have a shared history and culture, so the barriers to cooperation are low, and when problems do arise they are the closest and most knowledgeable people about them, and best suited to consider a response.
