I don’t run on the treadmill because (a) why tax your knees if an elliptical machine is right nearby? and (b) really, it’s more about my fear that I would be one of those runners with really loud footfalls — SLAP! SLAP! SLAP! — that annoy people. (Especially at the end of the session, when you start to get tired and sloppy.)
Besides, I think I had a good “run” on the elliptical tonight. At least my shirt tells me I did.