I liked Thrall a lot. I still like him. However, there are some very compelling reasons that he shouldn't be war chief again, the most relevant being that many of the Horde's current problems are partly his fault. In many cases, Garrosh just exacerbated what was already present.
Thrall's flaw was that he couldn't figure out how to really set the Horde on a new course. He wanted, in essence, the orcs to behave more like humans, which was good. However, in attempting to inspire the orcs with stories of "honor", he had to reach back and mythologize some pretty horrible people.
Now, this might not have been so bad if he'd gone far enough back into orcish history, to the actions of historical figures whose actions were too distant to really matter, and which could easily be changed to a more convenient political narrative. However, he screwed up by making Orgrim Doomhammer the model for orcs to follow. Whether or not Orgrim was a horrible person, most of Azeroth certainly remembers him that way (and personally, I don't care how reluctant Orgrim was to go along with the Old Horde; he still went along with it, and was a monster for so doing).
So basically you have these orcs, fresh out of the brutally mismanaged prison camps, possibly still going through withdrawal, and finally being told that they could be great and honorable. This had to be amazing to hear; the possibility of redemption can be incredibly powerful.
The problem was that their only models for so doing were: Orgrim (a cruel and rapacious conqueror), Grom (only slightly better), and Thrall. The problem is that the orcish past was probably never all that noble, and was still mired in an iron age mentality of might makes right, a mentality that simply doesn't fit in Azeroth's more developed society (where might still makes right, but also allows for aspirational values [which can be profoundly powerful, if slow] of something better).
The peons are another example. Maybe the lot of peons was better under Thrall, but it still sucked for them. A society that allows the vast majority of its population to be spat upon is never going to be very healthy.
What Thrall ought to have done is to make the orcs realize that, if they were going to be redeemed, they had to actually break with the past. The average orc's knowledge of humans was probably pretty limited, so Thrall could have probably enabled cultural transformation to a more peaceable state without raising too many eyebrows, at least back then when the orcs were still vulnerable.
Instead, he took half-measures. All Garrosh did was take it to the next step, which Thrall would have probably been unable to stop by Wrathgate.
Thrall's probably even less suitable now; by embracing Aggra's essentialist identity politics, he'll be even more about these harmful notions of orcish tradition.
So no; Thrall's time is done. He did the best he could, and given his situation, it was impressive. It's time for someone new, however. I'm thinking Vol'jin (who will be pragmatic without falling into Thrall's confused idealism or Garrosh's idiotic machismo) or Baine (who will actually try to be good).
Edited by Felya on 11/17/2012 3:51 PM PST