Until now, u couldn't twink lvl60 or 70 unless u didn't had the next x-pack, but now, after years of request, a simple feature will change the life of many, and let me dare to say that many more will adopt this new feature.
Farm that Van Cleef sword, and the full Defias Set without caring about XP gains. Whatever suits u!
But my favourite, and im tottaly in for it, is level a group of 4
For every mistake blizzard makes, they inject something that is wished, and thats why WoW can't be killed :P