To put plain and simple it isn't that the mechanics are harder per say, it is the how the fight is tuned. In WoW they actually tune raids properly so that they are supposed to take a while to clear. World 1st groups end up beating fights in way less gear than they were intended to be completed and usually end up class stacking the classes that perform well in that specific fight. If you look back at A4S and A8S you will see insanely large attempt counts as well. The thing is that the last 2 raid tiers have just been tuned terribly so you can literally just walk in and kill the boss with very little optimization and no gear beyond crafted gear. Even crafted gear wasn't needed this time around.