Too many things to try and address here, but to answer the original question in this thread:
The NSMBA, members of the riding community, Endless Biking and the DNV sat down earlier this year to go over trail classifications in preparation for the DNV's work on Fromme.
As Sharon mentioned, the trail network evolved organically and was not purpose built to a set standard. So, while we refereed to both IMBA and Whistler trail guidelines, we had to use some experienced judgment when assigning ratings to trails.
There was mention of rating a trail by the "crux" move, which really isn't that far off. Trails were rated based on the difficulty of the primary, intended line. So a trail Bobsled would be rated green/blue because most of the trail would be ridable by beginners, but a few sections jump to the intermediate level. Natural High is blue/black because most of the trail is intermediate, but there is one bridge (after the swivel teeter) that is advanced, and there are many advanced optional lines. Boundary would be considered double black because, although there are ride arounds, the primary line is expert.
Signage has a few purposes. Signage gives people new to the trail system and idea of what they are getting into. We believe people are intelligent enough to self select what level of rider they are, and if they don't they will quickly learn to.
It also gives all users an idea of who will be on the trail. So if you're a pro heading up to do 2 minute runs down Bobsled, you know you're likely to encounter beginners along the way and that the trail is intended for them. It also sets the tone for future maintenance and construction on the trail.
So for now the difficulty ratings are set, and probably won't be reviewed for a while unless there are major changes to the trail network, or an overwhelming amount of requests to change ratings of particular trails.