Logically there is a lot wrong with world government.
I recognize the advantages of centralization and hierarchy, for instance the fire brigade doesn't have time to decide everything democratically, if a car is parked in front of a needed hydrant they should have the authority to break its windows. The fire brigade is just a "rung in the ladder" however, world government would be inherently different as it is essentially the "top of the pyramid" in the hierarchy which is not answerable to any higher or competing authority. As a result the entire hierarchy would be subject to one political entity representing only a proportion of the total number of people under its authority.
Centralization of course also has its disadvantages, as with all things anything in excess is counter-productive and in this case world government would be an excess even in terms of streamlining diplomacy, world trade and issuing uniform standards, let alone problems with corruption, heavy hand of government and increasing distance of government from those it is meant to represent. Nations are perfectly capable of cooperating on their own accord without the need for some central body, the EU for instance was a success when it acted as a medium for negotiation then started to become inefficient when EU membership demanded compliance. Italian auto manufacturers are at the mercy of regulations passed by the Germans, British fishing fleets have to abide by Spanish rules, Greek financial decisions are influenced by Brussels and so forth.
tl;dr: centralization's disadvantage in taking into account a range of concerns is a form of administrative inefficiency