I am all for world peace and such, but I do not feel that a utopian, single-government society is possible with our race without substantial mental conditioning from birth and possibly even genetic manipulation. There would be many who would resist having this done to their children, and would either have to be eliminated or brainwashed. Much of the populace would resist if any of this had been carried out in secret, the genocide that would ensue wouldn't leave much left, and you'd have many people left who simply lacked the will to fight, and would leave not a utopian society, but the remainder of the world populace cowering in fear of an all-powerful regime. It's exactly the same that happens when a populace is conquered!

A forced utopia will NEVER be a utopia, it will simply end up being what you tend to recognize as modern Communism, as ideal Communism was a utopian society, which never occurred. The only way to get a utopian society is through the united efforts of every person involved with a 100% desire to make it work, which will never really work with humans as we are honestly too contrary to commit to our fellow man in that way. There are many people who would simply refuse because that's just the way they are or someone already involved is someone that they simply dislike for some reason.

Ignorance, greed, and pride are the biggest reasons why this would never work. Ignorance is very difficult to combat, and greed/pride are often parts of human nature because we've been programmed to survive in a competitive environment for millions of years. The only way to combat these are strict behavioral conditioning from a VERY early age, which would be essentially Brainwashing the young into this (an excellent reference for something similar would be the book The Giver), and would be very easily undone if they were allowed contact with the non-conforming Outside.

One could also argue that without true conflict, research could be stunted. There are many things that were researched and developed for War use that have benefitted the public well, especially in the Medical field. War is a terrible thing, but there are things we have access to now that we wouldn't have had if it weren't for our violent nature, heh.