I'm sorry I haven't been responding in this topic but I kinda forgot about it and I haven't got any e-mail notifications (and I haven't read all the replies, I'm sorry, but I tried to read at least most of them). Anyway, some of you wrote that challenge attribute would be better. I disagree, attributes shouldn't be bound to a single cache type. Like you have tree climbing attribute, but any cache type could have that attribute, but attribute "challange cache" would be just for mystery caches. I would expect that power trail should be an attribute, since there aren't only traditional cache power trails. Some one also wrote that there are many challenge caches in his region. I think that's just local problem.
D/T rating of challenge caches is also strange, I don't know what D1 challenge would look like... Maybe "find at least one geocache"?