why not a combination of ratings....
I really like the ski slope style of rating. The MTB trails I've been riding lately are all marked that way, and it is very easy to understand, even for someone who has never seen that style of rating system before.
The moab scale is very popular with the 4x4 crowd, but isn't well known to ATVs or hikers, or horses, or ????
You could have something like:
ski slope = Moab
green = 1 - 2
blue = 2+ - 3
black = 3+ - 4
double black = 4+ and above
it would be nice if the ski slope rating had an extra color or something.....
just thoughts