SPOILER ALERT! This video contains spoilers about 5.1, Thrall, Vol'Jin, you name it, if it's Horde lore, there's spoilers in this video. Watch at your own risk.
That aside, it's pretty interesting stuff. There are definite plot advancements taking place, and it's all very interesting if temporally rather troubling!
The return of Thrall, as incumbent Warchief, has to be a worry for Garrosh and his cronies. The footage is taken from one of the quest lines for the new Dominance Offensive faction. The new faction, and its Alliance sister, Operation: Shieldwall, are the result of the eventual arrival of Horde and Alliance ships dispatched by the relevant leaders from the relevant faction capitals. As the story goes, the Horde ships landed at the Alliance end of Pandaria, while the Alliance ships made an equal, yet opposite, error. Both factions wish to secure Pandaria as their own, and by carrying out quests for the two factions players can aid Garrosh and Varian in their quests to paint the new continent red or blue.
More spoilers after the break!