The trail runner's perspective (since I pretty much don't run on pavement)...

The biggest "problem" I see with the treadmill is related to the tendency to not vary stride, foot placement, weight distribtion, and the like. Now if you run on the road and always run on what I call a "reliable" surface, this is probably not a big deal (or no deal at all) because it probably allows you to fine-tune your form and efficiency. However, I find that (for myself) running on a traditional treadmill can be good exercise, but that it's no substitute for running on the trail in terms of balance, lateral movement, and form.

I think what a treadmill can/can't do for you depends on your running goals. And I think that not all treadmills are created equal (some decks are cusheir than others). I've gotten really sore running on a treadmill from what I interpret to be repetitive, non-varying movement. But then I've gotten really sore from running down 25% slopes, too.