All of my running on road, but as a city dweller not much choice anyway. I sometimes envy those who have country or coastal options (more for the scenery than the surface though, when I think about it).
I've ran on a treadmill once or twice, why I have I found I got very bored after about 10 minutes. There is only so long even I can look at myself in a mirror.
Fair point that you can't vary the gradient as easily but you can run on hills, maybe not in London but you can here. On a hill you can't just decide you've had enough and level it out you have to keep going to the top and even if you slow to a walk it's still hard going.
Mrs Squints auntie came out running with me once I asked her if she had done any hills before oh yes she said. Cheeky bugger then complained that I tried to take up the side of a mountain.
I'd never take a lady up the side of a mountian!!!