well, from my experience, on a treadmill, your training your body to "run" "jog" at a "set" speed, whereas if you ran outdoors, you run on multiple speeds that aren't really in sync, like "set"
in my opinion, treadmill is best for individuals who would love that extra push as your training your body to run at set speeds, whereas if you were outdoors, you might start to get lazy and run slow or spontaneously.
I like running on a treadmill :] , but it can't beat the feeling over breathing fresh air outdoors though :[