I have an arbitrary method of ranking trail difficulties. There are no real set parameters, but here is a rough guide:
Easy: Trails that take around an hour to complete, about a few miles or less, less than 1,000 ft elevation, easy climbs, paved or well maintained paths. Any trail I rank as Easy will probably meet all of these guides.
Moderate: Trails that take a few hours to complete, around 5 or more miles, over 1,000 ft elevation, steep climbs, or intense cardio. Trails I rank as Moderate will meet two or three of these items.
Intense: Trails that take 5+ hours to complete, 10+ miles, over 2,000 ft elevation, steep or slippery slopes, narrow paths, or require the use of ropes. Trails I rank as Intense will meet at least one of these items, maybe more.