This was my short answer:
"Definitely a lack of love. If we all truly loved the way Jesus taught us to love, there would be an immediate improvement. Actually, even if just Christians started to actually love the way Jesus taught, there would be an immediate improvement!"
This started to really challenge me, how can we expect non-believers to show the love that Jesus taught about, when a lot of people who say they are Christians can't even achieve that.
Most of the atheists go with the opinion 'Religion' is what is wrong with the world and if we weren't so hooked up on living in fear of a God that doesn't exist and imposing His rules on people, everyone would get along just fine.
I stand by love, and this is somewhere I fail big time, but I am trying all the time to love more of the difficult people I come across and remember that they are equal in God's eyes.
Romans 13:8-10 ESV
"Owe no one anything, except to love each other, for the one who loves another has fulfilled the law. For the commandments, “You shall not commit adultery, You shall not murder, You shall not steal, You shall not covet,” and any other commandment, are summed up in this word: “You shall love your neighbour as yourself.” Love does no wrong to a neighbour; therefore love is the fulfilling of the law."
We would all agree the commandments are a good moral grounding by which to live. Well as the Bible says, if we all loved, we would fulfil those commandments and therefore the world would be without murder, theft, jealousy etc and how could that not make the world a better place?
And if you're not a believer in God or don't think what the Bible says about love is relevant to the world today, that's fine - but you can't argue with The Beatles!