I'm not a big fan of rating systems. As a former competitive mogul skier and new whitewater kayaker, I've seen such gross variations in ratings that it almost makes them worthless. The only time I've found ratings useful are when used to compare relative difficulty under current conditions.

A single ski slope, for example, can range from ridiculously easy to incredibly hard depending on the surface conditions and mechanical grooming. Running a snowcat over my favorite bump run can change it from a "Black Diamond" knee buster to a "Green Circle" yawner. Mother Nature can dramatically change a rating in and instant. The run you skied effortlessly yesterday, under clear skies and soft snow, can be deadly when fogged in and frozen solid.

Whitewater river ratings can vary even more depending on water level. Unfortunately, a mistake judging the difficulty of a ww river can be deadly. Last fall, I paddled the Cartecay River in North Georgia at low flow. It was a very easy Class II. I paddled it again a week later after heavy rains and it became a VERY hard Class IV. The difference was exponential and I should NOT have paddled that river.

Mountain bike trails, however, seem to have less variation than ski slopes or rivers. Maybe a rating system would work better there. The difficulty, though, is in judging exactly what makes a trail hard. For a roadie, hopping on an MTB for the first time, a trail that's "tough" because it has long, non-technical climbs might be very easy for him. That same trail, however, to an overweight freerider who doesn't climb much would be very tough. A technical trail would be very hard for the roadie but a piece of cake for the freerider.