I could go on and on and ON about the merits of running on the road vs a treadmill but the reality is that sometimes you just can't get outside. The biggest difference is obvious - the road doesn't move itself underneath you. While that sounds like a duh statement it creates some significant differences. When you break running down, it boils down to 2 things. Stride length and stride frequency. Everything else influences these two performance elements either positively or negatively. While the treadmill works pretty well to help develop stride frequency if you are using it the right way it can be counterproductive for stride length in some ways.
On the road you anchor your foot on the ground and propel your body overtop of the ground. Stride length increases only because of increased mechanical efficiency or maintainable power. On the treadmill if you jump really high a whole mess of road will go underneath you without you doing much propulsion or at least in the right direction - forward vs up.
You can help to combat this by working at a 1% to 2% incline on the treadmill flat runs. You certainly will get benefits either way but nothing beats getting out on the road, track or trails and doing the real thing.