It is my considered opinion that the greatest opposition to Utopian life is conflict, and the greatest enemy of peace is humanity.
Countries. Perhaps the stupidest idea that any one ever came up with. Indeed, the human race might be radically closer to achieving our ages old goal of Utopian living if some ancient tribesman had kept his mouth shut about that bright idea. We'll call him.... Thog.
Here is where I send a silent plea to the Doctor to go back in time with a nice heavy rock.
Still, maybe it isn't Thog's fault entirely. It seems to be human nature to NEED to belong. It's a nice sentiment on the surface. You first belong to a family. You love your Mommy and Daddy and you want them around- you love your siblings even if they make you mad. Then you go outside your family and you make some friends. Nice, right? You care about all of them, you want them to be happy and safe so that you can be happy and safe with them. We love and we care. How very nice :D It makes us all warm and cuddly on the inside. Lovely!
"I love you, you love me, we're a happy family...." *Shudders*
But no matter how big this bubble of good will gets- there are always people who are the Other. The Others are threatening. They are scary. They have their own bubbles of people they care about- and they don't really care about your people very much. They want to have the best of everything, for themselves and for their own. The dark conclusion is that they will be perfectly happy to take from your people to make their own happier. And that's where fighting comes in.
Through history, the bubble has grown. Family, friends, tribes, cultures, nations and finally countries. And the unhappiness is compounded. Patriotism, the next stupidest idea ever invented is created.
USA! USA! USA! .......................... Nah.
For King and Country! Right? It's a great reason to kill each other in endless petty squabbles over chunks of land and trade rights. It's idiotic. Let's kill each other for tactical advantage based on what flag is flying over the dirt we are born on. Let's attack each other in bar fights for disrespecting each other's mighty empires of pointlessness. But in reality, is it truly pride in our patch of ground that motivates us to fight? Or is it all the money and the nice things it can bring us? We like money- even more so, we like having A LOT of money. We can care for ourselves and our own that way, right? We can buy them lots of things. We might delude ourselves into thinking that we can buy happiness and safety. Yeah, right. How is that working out for us?
"This planet has [...] a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movements of small green pieces of paper, which is odd because on the whole it wasn't the small green pieces of paper that were unhappy."
-Douglas Adams
So now we have established what I think is the problem- which is the biggest step in considering the solution. How to get rid of countries then? That is a thoroughly tricky question. We all love them so much. They are so very established in our society, in our minds - in some way in our nature. It might be very stupid but we need that bubble so much. So I say, make the bubble bigger. Like, atmospheric size.
Ahh, our beautiful blue marble.
We need to stop thinking of ourselves in restricting terms. Race, location, religion, it all needs to become incidental to us like the color of our eyes. We need to think of ourselves as Human- not American or whatever else you happen to be.
Unfortunately that's never going to happen freely. We will never give up our ties and ideas alone. We need there to be an Other. Peace won't come until the Other is somebody else. We need a common enemy or a common task to preform. Something that our survival as a race depends on. That is the only way that we will be able to think of ourselves as the same.
Eww.... Probes. No thank you!
So what now? Do we have to wait around for the aliens to attack so that we can have world peace and shared prosperity? Maybe. Maybe not. Perhaps the better answer is a common task. It might even be underway now. If we can advance technology far enough and fast enough, we might get to peace on our own. They say the world is shrinking and it's true. We can send about 50 different kinds of messages instantly across the globe, spreading ideas and making friends thousands of miles away- all from our nifty hand held devices. If our technology progresses far enough, it might eliminate want from our lives. Already we have three dimensional printers that can create essentially anything. How long before we have a printer in our house that can create anything we'd like? Food, clothes, jewels.... where is the value in these things anymore when every one has them ad infinitum? What will the benefit of war be when our economies and governments are so bound up together that damaging the infrastructure of one damages all? What will the point of separate government be when conflict comes to an end- when our interests are Earth's interests?

Is this what our gateway to Utopia will look like? Its this the vanguard of peace? Maybe. Maybe.