Warning, large post, also I swear a little.
The only things stopping me (and probably everyone else) from liking a one world state are:
1. Language barrier, we would need one language for the whole world, or methods of easy translation.
2. Culture barrier, even the differences between Canada and the USA are huge if you think of them. IE: Stance on guns, socialism, health care, race (Yeah, I went there, though we're not perfect either), and even the power of the government. IE: American states are way more powerful than Canadian provinces, the powers and importance of our leaders, etc. Then you go and compare USA or Canada to Russia, China...
3. Nationalism. I for one am very proud to be Canadian and better than everyone else, and I totally understand the same goes for everyone else and their countries. This is probably the biggest hurdle, there are very few people that can get past this. I know it's irrational, but I really just don't care.
4. Government. If you cut it down as much as you can you end up with three types: Autocracy, Democracy, Communism. I'm leaving communism separate from Autocracy just because I think they're different enough (when run properly, which is never). But then you have so many different types of each. Even under Democracy you have republics, constitutional monarchies, Athenian democracies, representative democracies, direct democracies...
If we were going to do this for really real this is how I'd do it:
One official (used by the government, for signs, etc.) language: A new language that uses the western script and is more 'logical' than English, to make it easy to learn, maybe not taking it as far as lodgban (logban?), but you get the idea. It wouldn't be the only language across the world though, English is the USA's only official language but plenty speak french, spanish, ect. This would be sorta the same. You could speak any language you wanted but knowledge of the 'new' language would be helpful.
As for the culture barrier, fuck it. There's enough difference even inside Canada or the USA, a global nation would have the same thing.
Nationalism: You can't really make this go away, the biggest problem is getting people to join. Ideally you'd have a referendum where everyone would vote on joining or not and you'd need between 60% and 80% to go ahead (75%?) other than that it would probably end up the same as the culture barrier.
Government: It would be a direct Democracy, everyone would get a vote on everything, there would be a (large) group of people that would suggest bills, but EVERYONE would vote on them. (Fuck logistics, that's what the internet is for). We would also have a constitution, but it would not include the right to bear arms, or similar conventions, everyone is voting so it should be less of a problem, if you want guns, vote for them. (Simplified I know, PLEASE don't start a flame war over this) You'd still have free speech, movement, security of person, etc.
Flag:
Military: NATO
People that don't want to join: Fuck em'
Poor countries that want to join: Let em' in, yeah, it's going to be costly to bring them up to speed, but fuck it, it's money from people to people to help people.