I train where ever I can, preferably outside, but I bet I run at least 1,500 miles a year on TM.
For me there are a few differences:
1 = Calibration: The YMCA has I go to has 14 treadmills and from the fastest to slowest calibrated TM I would estimate .8 MPH (A shame at a Y) so on the slowest TM you could have it set at 7.5 MPH and really be running 7.1 MPH and the fastest set at 7.5 = 7.9 MPH. It is humorous to me that if there are onlu 3 -4 people running when I show up, they are all on the slow treadmills (I guess a mental boost)
2) Incline / Wind resistance: You can calibrate a TM to be between 1.0-1.5% incline when it reads 0%, but most are not. You likely need to set your TM at 1.5% to make up for the "Belt moving you" and no wind resistance to = running outside.
3) Complete control: I like this part - You can trow in a hill anytime you want, I constantly am either making slight changes to elevation and sometimes speed to break up the monotony.
4) Bathroom close at hand:
I ran a 2 minute marathon PR at Houston 2006 on 2 months straight treadmill training with little to no outside running. If I had done the exact same training outside, I would have expected better results, but ... I find it easier (weather) to do some workouts on the TM and I likely would not have done them at all outside.
I agree, the more running outside the better, but you can be successful training on the treadmill. Use a minimum of 1.5% incline and make sure you use the nice elevation feature to do hill workouts