I played WoW since the tail end of Vanilla. A little over 7 years. And you know the worst thing Blizzard ever did?
No, it wasn't LFR.
It wasn't Cata.
It wasn't Timeless Isle.
It wasn't the plan to restrict flying in WoD.
It wasn't even keeping Shadow Priests at the bottom of the pile for so long.
It was removing the global LFG channel.
I never understood its removal. Losing that was the greatest wound, and still hurts to this day. I can't go around doing other things -- quests, gathering mats, rare hunting, none of that -- to make use of my time while trying to find a group. Instead, I have to sit around doing nothing in a city and use Trade chat (which isn't even meant for the group finding purpose).
The return of the global LFG channel would be a great "new" (old) feature for WoD.