The personal motivation of a great majority of individuals would be severely lacking.
Saying that, as geeky as Star Trek is, the original "utopian" vision of the Trek universe is actually not a bad one. it was very well realised in that they understood this hand holding vision for the future hippy types seem to have is simply not workable as more dont want it than do. We want to be able to better ourselves, prove ourselves to our friends and family. We do this through competition (of all kinds).
you can alos argue, the only reason the ST society works is because although the humans are united as one, the rest of the universe is not!
I simply cannot like everyone I meet, personally I try and temper myself so not to cause conflict with those people, other people prefer conflict to let the person they dont like know what it is they dont like about them, call it the need to dominate if you will. This very simple aspect of human nature (which lets face it is actually animal behaviour, its just we can rationalise it) negates the possibility that we can all just simply "get along". And this isn't being defeatist, it's being realistic.
Once people stop wishing for "world peace" and realise it's never going to happen, we can find alternative better solutions that work hand in hand with our own nature, rather than hoping every single human born in the future will have the same, non-conflicting nature. I actually think, given the scale of our race... we are doing pretty well on the whole...