I am working with a team on a sensor to give real-time feedback on running biomechanics, we are calling it Runnr. We think that runners who are just starting out and experienced runners who are tired would benefit from a way to be notified of poor technique.
We plan to measure where on the foot makes contact with the ground (heal-strike/toe-strike), impact load, and stride length. You would get feedback on these things through your headphones at configurable rates.
We are interested in how important these metrics are to you, and if there are any other things we should measure.