I would alter humans at the genetic level in the following ways. 1) Any group of people who go to war against another group of people would have an irresistible urge, regardless of the outcome, to execute their leaders/politicians as a punishment for failing to prevent the conflict even starting (that should end most wars and conflicts). 2) Humans would no longer suffer tooth ache, cancer, heart disease, back-ache or obesity. 3) The average IQ of anybody conceived after *NOW* would be 130 (not 100), and would increase by one point every 5 years. 4) The desire to have sex with children or adults against their will, would be removed utterly. 5) TV executives in the US would have an irresistible urge to bring back The West Wing, John Doe, Studio 60, Serenity and a new series of Star Trek (TOS), all using the very best writers in the business and, for the SciFi at least, an unlimited special effects budget.
Ok, so the last one's just for me, but what the hell?