This post exists because of the massive spoilers in this link, but the post itself will be spoiler free. As long as you don't click on that spoiler-heavy link, you will not see any spoilers in this post. (Edit - actually, there's one spoiler at the very end of the post - it's clearly marked as such, and it is a minor spoiler at best, but it is there. Let that guide your actions.) Instead, we're going to talk about the position of Warchief - how it came to be, how it evolved and then devolved, and how Garrosh Hellscream's reign as Warchief set the stage for what could be a completely new direction for his successor (whose identity I will not discuss).
The position of Warchief actually began as a complete figurehead, and the first orc to hold that position, Blackhand the Destroyer, was placed in that position due to his combination of physical fearsomeness and egocentric self-aggrandizement - so easily was he misled and directed by Gul'dan, head of the Shadow Council and architect of the Horde, that he never once proved himself a threat sufficient for Gul'dan to ever consider replacing him. It's not that Blackhand was either a fool or an idiot, he was in fact a canny tactician and a respected warrior. He simply believed his own hype - so convinced was he in his own superiority that when Gul'dan presented to him that he would be a respected equal and his position as Warchief would be one of real power, he believed it, because he believed in himself. Throughout the war with the draenei and later, the invasion of Azeroth, Blackhand ruled as Warchief and allowed himself to listen to Gul'dan's words - allowed himself to listen because they were telling him what he wanted to hear.
Even as the humans balked the orcs, and Blackhand's series of victories became defeats, he continued to listen to Gul'dan. This would be his downfall.