The closest answer given me thus far is that he will end the war of Iraq. It's possible, perhaps, though highly unlikely. He may well withdraw U.S. forces from Iraq, but the hostility, violence and bloodshed will remain. Who can say if it will decrease or increase. We simply do not - and cannot - know the future.
Even if violence in Iraq would decrease, the number of deaths throughout the entire duration of the war comes nowhere near the number of unborn babies killed in the U.S. alone each year (this is the most common argument I hear in support of Senator Obama; morals aside, taken logically, it simply doesn't add up).
It is a big issue here also, most Europeans though favor Obama, but I think it is the same problem - many of us do not know what he is going to change and what he stands for.