I've always wondered when talking with other people how difficult my courses are compared to what they are running. Elevation gain is one measure, but not always a good indicator of how difficult the course was. Steep hills can take more out of you than a gradual hill.
Would be great if Strava could categorize courses, or offer some kind of difficulty score. (GAP doesn't quite do it either). When watching the Tour de France, they have categories for specific hills. Would be great if you could categorize individual hills during a run, but better yet to categorize the entire course. Boston may be net downhill, but it's a lot tougher than running a flat course. Courses that have hills at the end would be more difficult than the hill at the beginning. There's a lot that goes into that determination, just hoping someone can come up with a good formula.
I just think it would be a very interesting conversation piece for people to be able to talk about the difficulty level of the courses they've ran. And for them to analyze which runs they have coming up.