If there was ONE thing you could change, on a GLOBAL scale...

[Morgaith]

I think i would remove all religions.

[Nirawen]

Rid the world of cruelty to animals.

[Stir]

Change the world economy to become a world-interest system rather than a self-interest one.

[Dezerte]

I'd make the world more empathic.

[spinner981]

At this point, I would select either greatly increasing the logical prowess of humans or greatly increasing the cognitive empathy of humans. I'm not sure which would be more valuable on its own. I suspect cognitive empathy would be, but I think increasing reason lends itself to naturally developing it.

An increase in logical prowess doesn't equate to an increase in logical correctness, and an increase in cognitive empathy doesn't equate to a decrease in conflicting beliefs.

If I could change something about the world, well aside from one of many obvious 'religious' answers, it would probably be for every human to know, beyond the shadow of a doubt, the truth concerning all things that humans can ponder.

Truth. Black and white. Correct or incorrect. Not some silly subjective opinion based on my own, or anybody elses, mere conjured up interpretation of reality.