Almost all hiking guides rate the difficulty of the hikes they describe. Frequently this is done on a scale like "easy - moderate - strenuous," where the criteria for the rating are not entirely clear. Some books, for example, will rate a long, flat hike as moderate or strenuous, because it's a lot of miles, but others won't, because walking on flat ground is pretty easy.
I think there needs to be a better system, so I propose a three-part scale. It has the obvious disadvantage of being complicated.
Part 1: The hike length. This is just the distance in miles (or kilometers, if you prefer). Users can judge for themselves whether 5 miles is easy or challenging for them. Guides already include this, of course; I simply propose that it not influence the other two parts.
Part 2: The total elevation gain. Basically, how hilly is the hike? This measure isn't perfect, as it obscures how the elevation is distributed. It's one thing to walk up a hill and then back down, but another to walk down into a valley than back up.
Part 3: The technical difficulty of the terrain. Here we have to get back into categories. At one end you have trails that are wheelchair-accessible. At the other, you have trails that require climbing ladders, holding on to cables, or crossing fast streams. Maybe like this: very easy = wheelchair accessible; easy = mostly smooth terrain; moderate = rock hopping, poorly maintained trails; difficult = cables, unbridged streams, etc.
I was thinking about this on Saturday, when we got into a discussion of how Virgin Falls compares to North Chickamauga. They're the same distance (4 miles). Virgin Falls has almost twice the elevation gain. North Chick has more tricky terrain. The trouble is, when we casually converse about this stuff, there are a lot of other factors influencing our judgments. When we did North Chick it was hot, and that was no doubt one reason I felt so tired. It's also hard to compare hikes when you did them at different levels of fitness. (Would I be as tired now if I did Bearwaller Gap as I was in the spring? Would I just be less tired because now I wouldn't get lost?)
The problem with a three-part scale is that it's not really satisfying. I think it's human nature to distill it down to one rating. So I suppose one could make a composite index, but then it wouldn't ring true for everybody. Someone who does a lot of short, steep hikes and someone who does a lot of long, flat hikes probably wouldn't find trail X to be equally difficult.