I was going to say empathy but there would be too many unintended consequences. We are already destroying Africa with aid programs as it is. Empathy needs to be tempered with more understanding to be effective. If I was offered the chance, I don't think I would pick anything. Our knowledge is not complete enough to fully comprehend what the effects of any such change would be, and we shouldn't be so arrogant as to assume we could.
If I was forced to choose though, I would simply make our species smarter. I could foresee it creating a lot of problems when 80% of the world begins to realize how much they are being screwed over by the other 20%. It could very well end up plunging Earth into another world war; but hey at least it would be a lot of fun seeing all the new things people would invent. I can't wait for the inexpensive space travel and robots. That or the apocalypse that arises when the underclasses lose all their jobs to the robots.
Lynx said:
If not that, at least remove that useless appendix!
The belief that the appendix is useless is outdated.
The appendix protects useful bacteria; the kind that produces vitamins we use, produces hormones for us to help store fat, aid in digestive processes, kill off bad bacteria, ect. When you get an infection that is particularly bad your body goes into overdrive and cleanses the intestines with metaphorical fire. Basically annihilating anything it can get it's hands on. The appendix protects some of the bacteria from this purging, so that it can repopulate your intestines. If you don't have the appendix, then you will face a high chance of recurring infection without the crucial bacterium it protects.