If I could wish for one thing, I would have to say equality. This could be on multiple levels. I think that if all people could live middle-class lives, with no one race, sexuality, gender, religion, or other beliefs thought to be more important than one another, many things like war and crime could be averted.
However, this sort of world would almost seem communist in nature, and in the past it usually ended up creating more problems than it fixed. So I might have to agree with Scarf in that I'd wish for a cure for all disease, as that would at least fix some of the world's problems.