If you've done both hard exercising - using truly challenging weights, pushing hard when doing cardio exercises, sprinting, dragging or pushing heavy sleds - you know this already.
But a recent study showed that on a molecular level, hard exercise creates different reactions within the body. The study is discussed here, on Gretchen Reynold's excellent blog on the New York Times website:
For Fitness, Push Yourself
Basically, if you push your body hard enough, it will create different, deeper, and more lasting changes to your body than if you don't.
This also reinforces the high-low approach that I like to have people use - push very hard on some days, interspersed with days of light, low-intensity exercise. No middle ground - either hard enough to trigger this kind of molecular reaction, or just some energy-burning exercise that gets your heart rate up a bit but not too much. Sprint, then walk, then sprint again the day after, instead of run-run-run. The science showing that there is a fundamental split between hard enough to trigger molecular changes and not hard enough would help explain why that approach is so effective.