Blizzard over the years has tried to emphasize "Bring the player, not the class". Which while is good for players in general, has its flaws. Imo, because of this design philosophy, many classes have lost their identity.
For those unfamiliar with this, let me explain. If we take a look back at WoTLK, every class had a niche, where some fights you really wanted this, and others, not so much. If we look at tanking, Druids were THE single target big hp pool tank, Prot Pallys were THE aoe tank. In healing, Holy Pallys were THE tank healers. While all classes could do everything, some classes just did things better than the rest, while being weaker in other spots. You didnt NEED a Holy Pally for tank healing, or a prot pally for aoe tanking, but it made it a lot easier. You didnt NEED a bear to soak certain mechanics, but it was easier with one.
I personally feel that this type of style is better for the game, and something I'd like to see again. Not to the extent of Vanilla, or even BC, but enough to make it so that some boss fights make you go "Damn, I can do that better than the others, my class is the best for this and that". And for me, it only made me love my class that much more.
Thoughts?
EDIT: TL;DR: A simpler way to explain it from the first comment "Every class shouldn't have a thing that makes them better than the other class, they should have something that makes them different."