Back in 2011, as a relatively new rider, I asked a similar question on the Mountain Bike Rider Forum (now defunct). The subsequent thread runs over 8 pages of A4. I have summarised some of the key points here. It's a bit disjointed, but most of the main points are represented.
I was primarily interested in why trails vary across the country and was there a difference to be found outside the UK. Surely there could be some consistency so that when you turned up at a Black trail, it posed similar challenges, or at the least required a similar skill set to the other black you did last week.
Typical responses were at one end: "How on earth can you do that when rider skill has a such a big part to play. What has my wife shitting her Lycras, I just cruise over at speed!" to "That old black trail would hardly make a red these days". That implies that either bikes or people had got better, which they undoubtedly have.
Then there is the impact of speed. Most, but not all, trails are easier if taken slower, but the fact that a highly skilled and supremely fit rider can piss all over a black trail does not mean that it should be down-graded.
The type of bike makes a difference, light XC all the way to a full-on DH monster. To say that the bike makes no difference would be insane. Which is why bike type should play no part in grading. If an expert wants to ride his rigid down a DH trail, that's up to him, he's only bringing some thrill to his ride. But they key thing is that he knows what to expect.
(In response to a named individual

"You seem to be saying that to beginners, all trails are black. Whereas to experts all trails are green. That may be how they see them, but it is not a valid way of rating trails. To try to take into account the riders’ varying skill and fitness levels is the way to madness. I would also not take into account what type of bike the rider might want to use. It’s their choice, they are intelligent people and can choose for themselves whether the bike they want to use, in conjunction with their own assessment of their ability, is suitable for that trail or not. In your example, the downhill section that would have you writing your will would be graded appropriately so that you would know in advance that (for you) it was a widow maker.
I have already made clear that I believe that the rider’s skill, fitness and ability are irrelevant to a grading system. It truly does not matter what skill etc the rider has. Let’s say that a rider wants to visit a new trail that is a challenge, without being overwhelming. The rider should be able to look at a trail guidebook and select any particular grade of trail and know that it will be of similar challenge
for that rider to all the other trails of that grade visited. Another more expert rider will find that trail easier. But so what? If they want a harder one, they can go visit it, or tackle the easier one faster or on a different bike.
(In response to a named individual

You are a policeman, so you will have a nose for a scene that sets your hackles rising. A newbie wouldn’t see it, but you do. Would you accept that it would be possible for you to tell the newbie what it was that you were reacting to? If so, then you are using your experience to make the subjective, objective. Surely grading a trail is easier than that?
I’m not trying to rewrite the laws of physics; I just believe that we can have a better grading system.
The whole point about a consistent, open and transparent system is that anyone with the necessary training could rate a trail and come up with the same score. And it shouldn't matter how "expert" a rider they were.