real running makes your glutes work a lot harder. You have to push against the ground and propel yourself forward. On the treadmill, the ground moves and you stay still. This works the glutes less.
If you always run on the treadmill, it sets the pace for you. Outside, you may not know how to pace yourself and end up running faster or at a higher intensity than you intended to.
Running outside is typically done on concrete, that is a much harder surface than the treadmill (unless you run on a track or a grass field).
i think treadmills suck ass though.