Personally I prefer Adventures that are too hard rather than too easy. Normal Karazhan was so easy it was a joke you could do it blindfold nor was Heroic very challenging.
The other problem is how does one create a difficulty that's a happy medium for all abilities, experience and card collections? Perhaps they could nerf it over time like they do with WoW bosses. If you really do give up there's always Hearthpwn and Youtube, there'll still be bosses that require multiple attempts before everything lines up though but that's the same for everyone.