First off, let me re-hash what is said in a million, zillion other places:
Treadmill Pros: Enhanced Safety, Indoors, You can watch T.V., Control of Environment (A.C., heated, etc.), "easier"
Street Pros: Burn more calories, change of scenery, exposure to sunlight, "harder"
Everywhere I've looked on the internet, it seems that the general scientific consensus is that treadmills are easier. So WHY do I have to run at a slower pace on the treadmill, and am constantly tempted to cut my runs short?
Because for me, it's all mental. When I'm running outside, I might challenge myself to run a little faster until I get to that tree up ahead. And I'm so far away from home, I have to run back otherwise I'll be late for work. On the treadmill, I constantly find myself looking down, and thinking, HOW MUCH LONGER?!? Music and T.V. watching isn't as interesting as being out in the world, people watching, looking at different houses, birds, and sometimes just getting immersed in my own thoughts.
I think a big reason why some people have trouble keeping a good running program is that they use the treadmill way too much. Of course it gets boring, of course you are grabbing onto those handles when you're getting tired (bad form!), of course you always have up-to-the-minute metrics on how you're doing. (BTW, I use the RunKeeper app on my iPhone to satisfy some of my data-hunger needs). So if you've been stuck on the hamster wheel for a while, I'd encourage you to get out there, and try a short run outside. You might surprise yourself how much you enjoy it.
Photo found on Flickr. SashaW. http://www.flickr.com/people/sashawolff/