To classify the overall difficulty of an MTB trail, I propose to use the commonly accepted blue/red/black color codes which map to a new tag on the mtb relations:
mtb:difficulty=easy|intermediate|advanced
The tag is similar to the accepted tag for ski pistes, the values are the same. See also my post on Talk:Mountainbike. The appropriate color coding code has been added to the site here.
This tag should not be mixed up with the existing mtb:scale tags which in my opinion are more relevant for the scales of individual ways of a trail. (this data is also rendered on the xctrails.org map, see the help page for details.