I think for XC trails only, difficulty should be a little easier to define. For example, the black trails at the mtb centers where I ride have nothing that could kill you (such as big drops or skinnies that require "circus acrobat" skills to negotiate) however, they require very advanced skills nevertheless. Even racers will walk some areas to avoid an unnecessary crash. The climbs can be killer too. There is no S3 type of terrain there.

So if I had to make up a system, I would certainly differentiate between XC, Freeride and DH, before talking about any level of difficulty. To mix XC with DH trails when discussing difficulty makes no sense to me.