You first meet Garrosh Hellscream crying his eyes out by a bonfire in Nagrand. I spent hours killing hundreds of ogres, demons, and more ogres liberating the Maghar because Garrosh was too much of a pussy to do anything himself. Next we see Garrosh in Warsong Hold in Borean Tundra. Not only does the SOB not remember all the shit I did for him in Nagrand... he treats me like dirt and once again I have to save his brown ass from all the scourge in the area!
Maybe I'm missing something in between there but it seems like Garrosh stepped through the Dark Portal and completely lost his mind. He went from a sad pathetic orc to flat out attacking the king of Stormwind in the middle of Dalaran. The word is that in Cataclysm he is going to become Warchief of the horde taking over for Thrall.
Thrall: Do you wanna lead this horde?
Thrall: Well... You can't... (shifty eyes)
Garrosh: Sure as hell do a better job then you.
(yes I shamelessly pulled that from a movie)
All of this makes me wonder though... from a lore standpoint what if Garrosh isn't really Garrosh? His father Grom was very easily manipulated by Mannaroth into drinking the pitlords blood. What if his son is just as susceptible to evil forces?
He was all fine in Nagrand and then BAM as soon as he goes through the Dark Portal gets mind controlled by an Old God. The Old God uses Garrosh to rekindle the conflict between the horde and alliance weakening them further so there is less of a challenge to Deathwing.