According to this NY Times article - http://www.nytimes.com/2011/07/19/health/nutrition/19best.html?_r=1&ref=health
- running on hard surfaces is better for you because it some how causes your body to relax more than running on softer surfaces. Too bad our ancestors didn't have any asphalt with which to pave over the world!
This is my favorite quote: Perhaps a runner who, like me, strikes the ground with her forefoot instead of her heel, might risk more injuries on softer ground. After all, every time I push off on a soft surface, I twist my foot.
Oh, the agony - I don't even know where to start. And that photo at the top makes my knees hurt just looking at it!