I'm a Northeast skier, and I spend most of my time skiing in NY. I've noticed over the years that what some mountains will call a Double Diamond, is no different than any other Single Diamond trail. Even on the same mountain, I've seen trails rated DD where the next trail over is nearly identical, but is a SD.
I always thought the difference was Moguls. A steep groomed trail would be a SD and a steep trail with moguls would be DD. I would probably expand that definition to include any ungroomed terrain and not just moguls. Early in the season there aren't any moguls anywhere. There just isn't enough snow yet, so I can understand that there would be little or no difference between SD and DD in that case. However, I've seen several instances where a SD trail has moguls on it, and the DD next to it is groomed. It just doesn't make sense to me.
I've been to mountains in the past where they didn't bother with Double Diamonds. If it was expert terrain, it was expert terrain. After all, if you are really an expert, there isn't much need to distinguish beyond that.
Is there a basis for classifying a trail as a Double Diamond? Slope angle? Moguls? Anything?