Does difficult imply fun though? It really depends on your perspective, for instance, Ultima was often considered an easy but fun fight. However, as a tank I've seen both the good and the bad from parties when it comes to the dungeons. You'll see some groups destroy it, and others struggle on basic mechanics. Personally I like them, they're difficult enough, especially the very last instance you do in the story. Encounters need to be designed with fun in mind first before difficulty.

That being said, I think they should make a challenge mode that does not provide gear rewards, some other form (to make it not feel like it's necessary or to devalue experts) should be there like vanity weapons/mounts/minions etc. It would be interesting to see dungeons tuned for max gear, or even pull a PotD and have no gear at all.