I reject the idea of a one-world government on the grounds of how unfeasible it is.
I do not think the world has enough common ground for the many different cultural, social and religious values to be accommodated into a single effective 'federal' plan. Even with high enough autonomy.
I also think that in a one-world government, it will become very easy to alienate the smallest groups of people. Given a OWG, China and India will have a voting power of 40%. How can the voice of the smaller countries be heard? For it to even work properly, the larger countries need to be fracture into smaller factions while smaller countries need to be expanded, something which will result in huge problems.