Running indoors and on the treadmill has been great for me. For a good part of 2008, that's how I trained. I spent plenty of time at the very beginning of my running year outdoor but once July hit I pretty much stayed indoors.
This year, I need to focus and concentrate more on outdoor runs. I will try and run at least twice a week outdoors.
Now, I'm not planning on giving up the treadmill entirely - how else can I do my intervals? But for my distance runs I definitely need to spend those outdoors.
Another element of running outdoors are hills. As I'll participate in the Mud Run again this year, I'll have to improve my conditioning in hills if I want to surpass my time from a year ago. But in general, running hills is a great way to build up the leg muscles and improve conditioning.