Well this is a loaded question. Every area has different level of measuring. I went back to Ontario and rode a 5 star trail expecting some difficulty. It wouldn't pass as a blue trail here (intermediate is blue). But here is my take.
I have only been riding for around 6 months (mayish). I found out quickly that speed comes later. Technique is something to be practiced and practiced. When my technique and trials skills picked up my speed doubled. That is definately key especially when talking about technical sections.
As for rating trails. I am almost a pure technique rider so I really can't help with distances and stuff. But for me single tracks are broken down like this
1a - tourist - wide fairly flat double track. Easy on the knees and boring to ride.
1b - beginner - fairly narrow with some roots and rocks thrown askew for some trouble. The odd 12' bridge can sometimes be found on these trails just to further develop technique. Usually 20k - 30k. The climbs aren't too long or steep. Sometimes this can be fairly easy fire roads.
2 - intermediate - More obstacles. Sometimes log rides (many logs piled up to make a bigger obstacle. Longer and narrower skinny rides. Narrower trail (running into tree happens). Small drops (can usually just be ridden and not actually dropped). Some wide switch backs. Still around the same distance but with steeper longer climbs. More complicated fire roads (esentially really long rock gardens) fall into this category.
3 - Advanced - More cascading obstacles. (obstacle flows into another obstacle) Sometimes very narrow with bigger drops (you either walk, fall or drop it). Teeter totters and other fun obstacles. Very rooty and rock gardens belong here. Distance can varry but if it is long it is technically tough, if it is short it is technically tougher and rougher. Swtichbacks are tight with berms.
4 - Damn that hurt - Everything possible. Uphill, downhill, technical switch backs. If it is a short trail (say 10k) it has a VERY steep incline (2200ft in 6k) and is very technical. Anything and everything falls in here and everyone falls and gets hurt at some point.
As you can see I am not a pure xc guy. I like it technical and tough sometimes bordering on extreme. Remove the obstacles and you end up having to go further and put in more cycling time. Definately not my thing. In either situation technical skill are always fun to learn
Needless to say it has taken me 4 months of solid riding to get to a comfortable level doing intermediate to advanced. This is riding 2 hours daily and practicing trials. Oh and I still think I suck. Just my competitive spirit. The minute I think I am good I stop pushing. There is always improvement.