I like the idea. But I think a lot of games that have this concept seem to forget that in real life countries can refuse to do what the UN votes on. You are just kind of forced into doing whatever they say, and some people like that I guess...but the fact that this game has an embargo option would allow for an alternative method of enforcement other than it automatically happening. So I say if you DO put a world gov in place, give people the option to defy their resolutions.
(Great example of being forced into it would be the (That terrible game that is totally irrelevant and I shouldn't be bringing it up anyways) senate)
That said I would prefer a UN type thing.
But going by what Sheepy said, Alliances kind of fill in the niche of a global gov.