It looks like we're all thinking along the same lines here. The comment about the kitchen sink is well worth considering.
I had a friend who was robbed on a train once when he fell asleep. They took just about everything, including his wallet and passport. He realised afterwards that the vast majority of stuff that was pinched was not necessary for him to travel with and he could have left it behind at home. The passport and wallet theft caused a big problem for a day or two, but it was all sorted out. Just take what you need, no more.
The people that insist on laptops, i-pods, a sack of camera equipment that weighs a ton, expensive mobile phones etc., really need in my opinion to re-think the reason they are travelling for. All you need is a few clothes, sleeping gear if you are camping, a good compact camera, basic mobile phone and a few knick-knacks that are of necessity rather than unnecessary luxury. It's a proven fact that the vast majority of inexperienced travellers take around 3-4 times more gear with them than what they actually use or need. Overpacking could be your only dilemma.