Although I've never run on a "dreadmill" in my life, I would say that the main difference lies in the fact that when you are outside, the ground isn't being pulled along beneath you, thus making the job the legs have to do that bit harder.
... Then of course there's headwinds, hills (up
and down), uneven ground, rain, grass, mud, traffic, dogs/dog owners... etc. That's
proper running!
Then there's the fact that if you're running outside and suddenly feel like you've had enough after a couple of miles, you can't just press a button and step off... you've still got to get back to where you set off from. Or maybe that's the advantage that some of the gym "hamsters" are looking for?
OK so I can't compare outside to inside; but I simply can't see the attraction in the dreadmill monotony.
