every nation has divergent views which serve only to divide not unite.
Ehh... I wouldn't agree with "only", but maybe I'm just being nitpicky.
But I do agree with your general point: the U.N.'s power is often compromised due to its members' (often) inability to function together.
culture and diversity would become dead,
Why do you say that? Just look at the U.S.- run by one federal government, but plenty of diversity between states, and even between cities.
Now on to the actual question:
There is nothing inherently wrong with having a world government. Well, there is nothing inherently worse about a world government as opposed to a "less than world" government.
What do I mean by "inherent"? I am referring to the basic concept of a world government. There is no fundamental trait of the concpet of governing that makes it "worse" when you govern 3 people instead of two. Now, if you think that any and all government forms are bad, then obviously you might disagree with me. But, even if this is the case, larger governments wouldn't be bad for different reasons than smaller governments, it is just the same reason but on a larger scale.
In other words, if we have 4 people governed by two governments (two people each), how is this worse than if they were all governed by the same organization? They are all still being governed. If the purpose of government is to keep people acting in a civil manner (whatever that means), it shouldn't matter how much you divide up the population.
In fact, it is guaranteed that there exists one government that is at least as efficient/fair/whatever as all the others. So, replacing all the other organizations with this one could only improve things.
BUT.
There is that old "good only on paper"* thing. In the real world, people have had their own governments and cultures for a very long time. Just because the Mexican system is more efficient in Mexico than the Cuban system is in Cuba doesn't mean the Mexican system would be more efficient in Cuba. Because, well, Cuba is not Mexico.
Then, there are economies of scale. I'm not sure if this is really applicable to governments, since many are already on such a wide scale that increasing their reach probably wouldn't have a great effect on efficiency. But it should still be considered. Maybe a World government would be too bogged down in its own bureaucracy to actually function.
But even with these issues, I am sure that there is some sort of world government that would largely solve all the issues of war, immigration, importation/exportation, etc. In this age, the entire world is our base of operation. Some way of coordinating efforts would go a long way. But, I don't think that any current government is fit enough to take on this problem. I am not sure what form this system would take.