Garrosh Hellscream is probably one of the most contentious additions to the Cataclysm expansion. Oh sure, he existed all the way back in The Burning Crusade, but he wasn't Warchief at that point in time. Nobody thought for a moment that the depressed Orc in Garadar would one day be leading their characters into battle against the Lich King. And nobody dreamed that Garrosh would one day be sitting on the Horde's throne.
It wasn't so much the surprise of the situation that people took issue with, however. Instead, it had much to do with Garrosh's attitude throughout Wrath of the Lich King. Headstrong, ruthless and downright bloodthirsty, Garrosh's decisions in Wrath didn't ring right or true with the playerbase. Even more jostling to players was the notion that despite the player dislike of the character, the NPCs of the world loved the guy. It may very well be the first time that a large chunk of a faction's players have blatantly disagreed with a fictional NPC faction -- and the NPC faction won out.
The Shattering makes it clear that Garrosh's position is only temporary, but in Cataclysm, he's certainly attacking the job with gusto.
Today's Know Your Lore contains pretty much every possible spoiler that exists for the novel The Shattering by Christie Golden. If you're avoiding spoilers, run away! Run away!