I do not strongly support a US hegemony or empire ~ I find the idea of a Pax Americana almost as off putting as global communism, because power corrupts and ultimate power corrupts ultimately ~ the only reason that the US in it's current condition is not the same as a global government is because it doesn't have total control over everybodies lives and it is scrutinised and held to account by it's citizens. I do support the idea of free nation states built on principles of liberty and democracy acting in their countries national interest. Now has the US protected these principles all the time ~ of course not, it has put it's interests above the rights and freedoms of those in other countries heaps of times (just like almost every other country, Australia's relationship with Indonesia after they invaded East Timor a good example). But if it is taking actions that do furthur the liberal democratic model that has worked so well around the world spanning vastly different cultures and societies today then I think that supporting those actions is the right thing to do.
I am fearful of authoritarianism, I see how much evil is comitted when the power is consolidated ~ look to communism and the 100 million people that that system managed to get killed ~ it is not a good idea, the dictatorship of the proletariat only means violent aquisition of wealth by the state and minimal redistribution until everybody is a slave to the state, look to Nazism and see the police state, big government interference and control over peoples lives genocide brought about by a madman, look to political Islam and the crimes that are comitted against innocent people for doing things like having pre-marital sex or not submitting to the religious authorities who are infalable and who's decrees come from a higher source and are thus unquestionable. These are all bad systems and they all share a lot in common ~ there is consolidated power, there is a fearful and supressed population, there are leaders who are venerated and build up cults of personality. When your fundamental freedoms are picked away piece by piece the political system usually winds up in an extreme position.
I see where you come from in your question about the US ~ if you keep your eye on the other big bad then you are blind to the more insidious evil that seduces you to surrender the freedom to act and ultimately to think. That is a significant issue. The only way to prevent it from hapening is to have an open political discourse where all issues can be adressed without censure, where the power rests with the citizens and not the few elite who know what is better for the majority. The USA is a liberal democracy, it has different tiers of government and a seperation of the powers, it's citizens are armed (frankly that makes the US more of a free country than Australia in one way), it has a bill of rights. It has all of these different checks and balances that have ensured that it could never go to the extremes that were found in countries that lacked all of those guarantees. I am not saying that it is impossible for the US to become a Nazi Germany ~ I am saying that now it is only as authoritarian as any free liberal democracy with a free press and freedom of speech is. It is built on the backs of many cultures and races so the idea of a leader rallying the people under a unified racial alliance seems very unlikely. I think that when the US is in such a state then we will not hear about the problems at all and criticisms will fall on deaf ears in the state censored media. People will disappear ~ not like Guantanamo where they are captured in hostile environments with reason to believe that they are a threat, listed, get legal representation to take their cases through the courts lobbying their governments with ICRC visits and media attention. I am talking bullet to the head and into a mass grave along with an entire family with everybody too afraid to take action type of disappeared.
I would love to see a world government built on those same principles ~ I think that it is a possibility when those free nations learn to work together through alliances and trade. For any world government to operate effectively people would not be surrendering their rights ~ they would retain them. You would need to have a united humanity ~ this would require less religion in the world - something that I do not think is going to happen naturally until poverty can be alleviated, education becomes a universal human right and living standards are brought up to scratch. Religion is a big problem here because frankly it is a retrograde force in the world and for humanity ~ it divides people into groups, it elevates worthiness and crushes the importance of the individual. I would not want to violently supress religion, that is not how to adress it. People need religion when times are tough and it is difficult to survive; I think that there are plenty of examples where once people are not living in fear their entire lives or dying at 35 from preventable disease and they have higher living standards and education that religion becomes less influential ~ spirituality remains important but institutionalised religion just doesn't hold as much sway over people. It is an obstacle to world government that would be overcome once all the other innumerable problems are.
I would hate to see a world government where the people are enslaved ~ where they have to pay very high taxes to keep most of the world stuck on global welfare dependence, where technophobia obstructs beneficial technologies from adressing global problems (thinking of genetic modification of foodstuffs and nuclear power ~ two breakthrough technologies that could do so much for humanity). Where there is ultimate power in the hands of the few or where corruption is rife ~ that is the problem with the UN, when you mix ice cream (liberal democracies) and shit (autocrats, theocracies etc.) together what comes out isn't halfway, it still tastes like shit.
If you were to show me a world government that worked like The Federation from Star Trek then I may be inclined to be impressed ~ every person being able to live their lives freely, no poverty or hunger, material needs met through a near magical technology the replicator (chief issue; the federation is almost a working communist system in nature; it lacks a monetary system, every persons needs supplied by the government ~ the problem with communism is that we don't have replicators and we need people to work the crappy jobs and get the short end of the stick) and humanity is dedicated to betterment of itself. If on the other hand the world government was made up of a bunch of corrupt beurocrats with their own agenda's punishing certain groups in one way or another for the greater good then it would be less desirable and more akin to what I imagine this single world leader doing whatever they want is.