I experienced the whole Garrosh story more as a sidestory compared to the rest of the expansion.
Yes, there was always this Alliance vs Horde conflict in the background and this also introduces us to pandaria in the starterzones, but after that I never looked back to this. Not even the Landfall patch could make me feel like I was focussing on the "old struggles", I just did the quests for Valor back then.
I feel that the Sha, Mantid, Zandalari and Mogu (and ofcourse all the other races on pandaria) were the center of my Pandaria adventure and story.
And this eventually leads me to conclude that, in my personal experience, Lei Shen was the endboss of the Mists of Pandaria End boss.