They aren't exactly a hard class to master... what would be a mechanic that is overly difficult to master, compared to say a fire mage...
No, warriors are a joke in terms of difficulty compared to what they were in LK and even Cata. They have been given far too much mobility, utility, and damage. They were widely regarded to be a class that everyone rolled in MoP simply because, compared to say a subtlety rogue, they require virtually no skill to play well...
This is what we call a fast learning curve and a slow skill cap. You essentially master the class within a few days.
Compare this to a fire mage, you will take ages to actually be at a place where you are decent, and will likely never master the class (slow learning curve, high skill cap)