Have he lost his power? He still summons bridges of earth in legion and in the sanctum of dominion. I think it's more of him refusing to WOW TBC Gold use the powers he used to kill garrosh.
He feels guilty He isn't ashamed. He mentions it in conversation with Dr. Drake in korthia. She assures him it's not the fault of him.
The internal thrall is split between whether it's his fault or not. He claimed that Garrosh was the one to blame when he killed garrosh. However, he was not happy that they led garrosh along the same path that led to those choices. I believe that cheap Burning Crusade Classic Gold we will have thrall returning to his shamanistic self after this conversation with drake in korthia.