Although I'm not the most ardent fan of Garrosh myself, the following article entitled How I learned to stop worrying and love the Warchief brought up some good points.
Sure he's made his fair share of mistakes. He might be twisted, cruel, malicious, and downright delusional. But you know what? He's made for an interesting character. And shouldn't that be what we as players thirst for in the game?
You could argue that his turn from emo crybaby in Nagrand to the baleful dictator he is today came a little abruptly (much in the same way you could argue that Anakin Skywalker's fall was too abrupt), but what better juxtaposition is there to the calm, sapient rule that Thrall enacted?