Boy, Ragefire Chasm, something I haven't heard about in ages.
I'm just gonna let Wowhead's own Ashelia speak for me here:
"In short, Ragefire Chasm is redone. Trogg corpses litter the molten floors of the instance--they aren't the enemies anymore. Instead it's a new cult, former members of the Horde called the Dark Shaman. Garrosh has ordered you to clear out Ragefire before they can become another Twilight's Hammer and it's a pretty obvious lore point calling to Garrosh's eventual overthrow."
Completely new bosses and trash mobs and a much more trimmed overall Chasm? Count me in.