Here's an argument I've used a few times:
A basic law of physics states that an object in motion will stay in motion until acted upon by an external force. In a collision, occupants of a car are thrown forward until something stops them. For a forward facing child, the 5 point harness stops his body, but his head continues to move forward, causing his immature spine to stretch. A young child's spine only needs to stretch 1/4 inch to break, but in an accident it can stretch up to 2 inches. This can cause severe injuries, including internal decapitation. (include link to joel's journey) 90% of those who experience internal decapitation die from their injuries. When a child is rear-facing, his body is cradled in the seat and moves forward with the car seat, protecting his neck and spine from injury. This is why it is 5 times safer to keep children rear-facing until at least 2 years old, but preferably to the limit of their convertible seats. The laws in most countries are the bare minimum requirement for forward-facing. Aren't our children worth more than the bare minimum? Keep your child rear-facing as long as possible!
Here's a very simple experiment to show why RF is safer: Take a small child's sock facing away from you and hold it in your hand with your thumb. Move your hand quickly forward away from you and see how the sock flies forward. Now, turn your hand around backwards with the sock facing you and move your hand away quickly. The sock barely moves. This shows the difference in forces upon a child RF vs FF.