Challenging is a different animal for each kind of class. Something trival for tanks and DPS may be challenging for healers. Something difficult that requires you to keep NPC associates alive may be easy for healers and difficult for others. Blizzard's business case is to create a lot of content that average players can conquer. I suppose that may be because there are so many of them.
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Hence my point about weekly chests that benefit open world play vs. chests that benefit dungeons/raids. Surely, some creative genius at Blizzard can imagine some gear attributes and affixes that work best in separate environments.