Unfortunately, this is the most salient point in the original post.  Everything from mana cost to tank damage are designed around having the correct gear. Doing an 85 zone in 83 gear is much, much harder than doing a 75 zone in 73 gear. Why? Because with only 5 levels, we made the level to level jump in item stats really high. That also means mobs also jump up a lot in power from level 73 to 75.

Almost every “Wow, things are hard” thread I have read on this and other forums are a direct result of not having Uldum and Twilight gear yet. We thought long and hard about whether it was a good idea to bump the level cap without that gear in place. In the end, we thought it was worth getting feedback on the quest design and some of the encounter design. But encounter balance is enormously dependent on having the right gear. Just keep that in mind. Things will feel better when you have ugrades available. (Source)