The treadmill belt offers some help by pulling your feet back underneath your
body, so you are potentially exerting less energy to move your feet and legs
than if you were not on a treadmill.
When you run indoors on a treadmill, you do not have to overcome wind resistance. The lack of wind means you'll spend less energy running four miles on a treadmill than you will running four miles outdoors. However, it may depend on how fast you are running. For the average person, running 5 - 9 miles per hour (mph) will result in little difference. Some studies say outdoor running expends up to 5 percent more
calories; if you run faster than 9 mph, running outdoors could utilize up to 10
percent more calories because you are working harder against wind resistance.
Other studies say there's no difference. One study demonstrated the way to
balance energy use between indoor and outdoor running is to set the treadmill at
an incline (or grade) of 1 percent.
A few good points that I found on a running site explain it the best...
http://news.runtowin.com/2006/07/20/track-vs-treadmill-whats-the-difference.html
Treadmills are softer. You are going to take more of a pounding on asphalt than you would on a treadmill. If you jump into it too fast, you could hurt yourself. However, running on a grass or dirt trail could be even less impact than on the treadmill while still allowing yourself to stretch out a bit and avoid the boredom.
Running outdoors encourages stronger bone growth. Because there is more of an impact, there is more pressure on your muscles and bones which will lead to faster results when you are training.
You don’t have to break the wind on a treadmill. While you will not spend as much energy pushing air out of the way as you might on a bicycle, it still constitutes a large percentage of the energy that you are using. Air may not seem like much, but it takes a lot of work to get it out of your way so that your body can pass through it, and this is only made more difficult when there is a headwind pushing that air towards you. On a treadmill, you aren’t actually moving, so you don’t have to push nearly as much air out of the way.
Running outdoors gets you somewhere. Running is meant as a means of transportation from one place to another by moving over the surface. Running on a treadmill is having the surface move beneath you. It is not quite natural.
Running outdoors is more exciting. I have a definite bias against running on treadmills, mostly because it is so boring. When you run outside, you need to pay attention to where you put your feet, avoiding traffic and other pedestrians, and enjoying mother nature. There are more distractions that can keep you from dwelling on how tired you are, and to give you something to ignore when preparing for a race and learning how to focus.
Treadmills do not allow you to run downhill. At least, I have never seen one that does. For proper working out of the muscles, you need to work downhill and uphill workouts into your training. Always going along a flat or slight incline is not a natural way to move or to train.
The 5 most common treadmill mistakes do not apply outdoors:
Taking it too easy
Stepping off of a moving treadmill
Using the
handrails
Unnatural stride length
Not realizing how much easier it is
Treadmills are safe from cars, but you are not likely to get thrown off of the ground like you can from a treadmill. Getting hit by a car and being thrown by a treadmill are both experiences I do not really care to repeat.
It is tougher to hyper-extend your back outside than on a treadmill. If you do not use at least a 1%-2% grade on the treadmill, you can wind up leaning back as the belt kicks your legs backwards and you can hyperextend your back and hurt yourself. Using a small grade on the treadmill forces you to lean forward, which helps to protect your back.*
