Ultimately, everything that has happened under the tyranny of Garrosh Hellscream falls on the shoulders of Thrall. Even though he is considered a selfless hero, his decision to give the mantle of Warchief to Garrosh was a serious lack of judgement, discernment, wisdom, and (I believe) flawed moral character.
Besides the CLEAR signs that Garrosh wasn't ready for the responsibility (his attacking Varian at a diplomatic meeting in Dalaran, his behavior during the Trial of the Crusade, and his challenging Thrall after taunting his leadership ability), Thrall ignored critical advice and council from his most trusted friends. Vol'jin warned him, Cairne warned him, and Saurfang disagreed as well, yet Thrall was either too proud or too blind to recognize the serious mistake he was about to make. Even after he gave Garrosh the mantle, Voljin and Cairne expressed serious concerns, which Thrall ignored. Thrall should have corrected his mistake before he left for the Maelstrom.
So, why did Thrall ignore the friends he trusted and make a devastating decision like this? My answer would be Thrall's own selfish ambitions. One thing you learn very early on about Thrall is that he had a deep affection for his fallen friend, Grom Hellscream. What would bring Grom more honor than for his own son to lead the Horde that he died to save? I believe that it was Thrall's desire to honor Grom's memory that blinded him to the serious danger that Garrosh posed to the world.
Yes, it was Thrall's duty to leave Orgrimmar and save us from the Cataclysm, but of all of the options he had for a temporary Warchief, he chose the worst possible candidate. Voljin, Carine, Saurfang, Nazgrim, and Etrigg would have all been obviously better choices than Garrosh. But again, Thall was blinded by his emotional need to honor Grom's memory.
The genocide of Theramore, the destruction of the Vale, the death of Cairne, and for every other act of savagery that Garrosh has unleashed on Azeroth, the blame falls to Thrall.
In my opinion, he is a fallen hero who owes a serious debt to the people of the Horde, the Alliance, and of course the Pandaren.
As a Horde player, I firmly oppose any further influence that Thrall has in the Horde, especially in selecting a new Warchief.
Thrall has unwillingly become a villain this saga, even though he had good intentions.
Consider how many lives would have been saved and how much destruction could have been prevented, if Thrall hadn't failed so miserably despite the trustworthy advice of his friends.