Originally posted by: Skacer
I think wikipedia sums up the faults of a treadmill nicely:
# Lack of wind resistance makes running on a treadmill easier than it would otherwise be on an equal elevation grade outdoors. Training for outdoor races is complicated due to the subtle differences.
# Ordinary treadmills, even the expensive models at gyms and clubs, are not usually able to tilt at the steep slopes used on medical treadmills for standardized stress tests.
# Imposes a strict pace on runners, giving an unnatural feel to running which can cause a runner to lose balance.
Outdoors is only better in scenario 1 and 2 if you are training in specific conditions. If your run actually goes through some very steep hills or you run on significantly windy days. Otherwise these aren't even differences. The only major one is the 3rd one, and I find it more of a pro for treadmills, because it keeps you from slowing down. If you train hard on a treadmill and become a good runner, you will do fine outdoors as well. Infact, these cons are really only relevant if you are training for some sort of professional running, not just getting into shape.
If you can keep from getting bored on one, treadmills are excellent.