I don't think this is a necessarily bad place for this discussion, but I'd say the problem with world peace is the conflicts between

1. The definition of peace
2. How we perceive tragedy
3. How we react to tragedy
4. The struggle between those who want peace and those who aren't the least bit interested
5. The very real possibility that the most peaceful society is the least desireable society

My gut tells me that peace is in direct conflict with human nature on a very biochemical level. We have a system of rewards built into our brains. For a dopamine response to have any effect, though, there must be a withdrawal. Consistent rewards have diminishing returns on positive responses, not just with humans, but all animals. Intermittent reward systems have found to bring the most positive responses, but can cause extreme disparity and even self-destructive behavior. Negative reinforcement also has diminishing returns, and many times even causes circumvention (breaking the "rules" to avoid pain... ).

So, from these big-three types of reinforcement systems (there are more but they are derivative of these in some fashion), we can posit three societies:

1. Society is very free and giving, but some people will seek pain in themselves and others to appreciate the positive responses they are being fed (pleasure from pain).
2. Some people get what they want some of the time, but we will need to "balance the scales" when they tip...war being one method to do this (pleasure from struggle).
3. Society is very constricting and those who find seek some happiness will likely have to violate society's rules to do so (pleasure from dissent).

I'm sure there's other options... but the point is... the possibility of world peace seems... bleak.