I dig the "S" system, with only one nit to pick: The grade percentages are pretty steep! A 40% grade is very steep, and 70% is pretty rare unless you're riding down cliffs. Trails just don't stay trails at that grade.

Over what distance were you considering these grades? I notice that you have "no drops" with the 40% grade section. I know of many trails with very small drops (all < 1') but almost no sections at 40%.

Anyway, these ratings tied to obstacle type and trail condition are good. How do we evangelize?