WARNING: The following post contains spoilers for World of Warcraft: Cataclysm. Players who wish to play the new expansion spoiler-free should veer away from this post.
Please note I've put a spoiler warning on this post. This is because the following content, while mostly sheer speculation, may or may not end up being correct and will also directly address several rumors regarding Cataclysm that have not yet been confirmed. If you see a "TFH" demarcation on any future Know Your Lore posts, these are "Tin Foil Hat" predictions based on current lore and are in no way actually indicative of anything officially from Blizzard in regards to the game or where it's going to go. If anything presented here does end up being correct, these will actually become Cataclysm spoilers; if not, we've still had plenty of fun trying to predict how things are going to go down! Potential spoilers start immediately after the break.
The above image of Thrall and Garrosh was used in all five Horde politics posts for a specific reason -- when Cataclysm hits, Thrall is taking off and leaving Garrosh Hellscream in charge. This was confirmed in World of Warcraft The Magazine issue 1, page 19. Garrosh Hellscream is the subject of much heated debate among players, but he's also the subject of much contention among the Horde in general, and his actions will do much to shape the future of the Horde.
For the orcs, it's a matter of a "real, proper" orc from Draenor coming to Azeroth to lead them back into their former glory. No more talks of peace and negotiation, no more hand-holding with former enemies. Garrosh is the sort of leader who leads by action instead of words. The orcs at large have been slowly growing discontent with Thrall's leadership for that very reason -- despite all his words, nobody's been listening to them. To the orcs, Garrosh represents the face of what they'd like the Horde to look like – or what they think they'd like the Horde to look like.
It's the same tendencies that Garrosh displays with every conflict he's presented with, the annoying habit of reacting before thinking when presented with something that he doesn't agree with. However, given Garrosh's tendency to explode at any given moment, given the penchant for unwarranted attacks and violence of his forces in the north, given his extreme hatred for Varian Wrynn and the Alliance, it may be possible that Garrosh is being set up.
Oh, not by anyone in game. By the writers in charge of the story. Garrosh is a perfect example of the "grass is greener" philosophy. When given a leader like Thrall who promises change, people will eagerly flock behind that leader in droves. When that leader fails to deliver that change as speedily as people like, they grow impatient and wonder if this magical "change" that was promised is ever going to actually occur (or if the leader is just full of hot air). So bring in one of the old guard, the leaders who did things the "right" way in the past. After all, the grass is greener over there. What's the worst that could happen?
It's my theory that Garrosh is being brought in for two specific reasons: to give the Horde that savage, war-based background that they've been lacking since Burning Crusade, and second, as a highlight to show exactly how terrible those old ways were. To show that Thrall does indeed have the best intentions for his people, and as an example to the orcs that approved of Garrosh. By giving them what they "want" -- an orc prone to violence, brutality and acting before thinking -- they'll be highlighting what the orcs are missing with Thrall gone. Whether this means that the orcs will actually realize that Thrall is the best leader for them, or want him to return, is another story altogether.
Where does this leave Garrosh, though? The kid who's just trying to live up to the massive reputation of his father and lead the way he thinks things ought to be going? I don't necessarily believe that Garrosh is a terrible orc or a terrible leader -- but he isn't an Azerothian leader, he's a Draenoric leader. While Garrosh has been quietly helping Greatmother Geyah in Nagrand (and wallowing in a self-indulgent case of the doldrums), the orcs of Azeroth have been evolving beyond him, possibly beyond his comprehension. Garrosh is trying to do what he believes is right, but those beliefs are based on the leadership system of an old world that is dying off. In a way, I almost feel sorry for Garrosh, because the way he was raised and the beliefs he's always believed in simply don't apply in Azeroth, yet he's been plunked in the middle of all of it and expected to simply adapt without question. It's about as ridiculous as bringing a caveman to present day and asking him to use Twitter.
Regardless, on that world, his homeland where everything is "proper" according to his beliefs and standards, there are no trolls, tauren, certainly no forsaken, and blood elves are a recent pest. Given that, it is extremely hard for him to relate to any of the Horde's allies because he's never seen anything like them before. However, it's worth noting that in the article referenced above, the development team also makes the startling statement, "While Garrosh is a tough leader, he does see value in all the races that belong to the Horde, and he'll use them to the best of that value."
Let's go with the forsaken first, as they had a pretty large presence in Wrath of the Lich King, and the events at the Wrathgate marked the first moment in WoW's history that the forsaken's plans were made utterly, brilliantly, crystal clear to both Horde and Alliance. Sylvanas, despite being nearly "murdered" in the attempted coup, is in the doghouse as far as the rest of the Horde are concerned. In Garrosh's eyes, a true warchief would never partner with cowards -- and Sylvanas' flight from the Undercity, her request for aid and apparent inability to predict what her people were planning right under her nose all clearly designate her as an incompetent leader who is not to be trusted from Garrosh's point of view.
In addition to this, Sylvanas is dealing with the loss of her motivation for moving on. Arthas is dead, and Sylvanas now has nothing left to do. Vengeance drove her this far, but with the absence of that object on which she could focus her rage, her turmoil, we are left wondering what exactly it is Sylvanas will be doing. When players turn in the music box that the Lich King had on his person, Sylvanas says "But what now, Hero? What of those freed from his grasp but still shackled to their mortal coils? Leave me. I have much to ponder." In the novel Stormrage, the nightmares of Sylvanas involve being brought back to life and then tortured by Arthas, again and again. The torment never ends for the Queen of the Forsaken, even if the torment is largely internal.
All of these things are going to make her upset, and when Sylvanas is upset, she gets angry. Fortunately, with the appearance of the worgen, she has something to take her anger out on -- either that, or by attacking the worgen and attempting to take over the land that they hold, she hopes to redeem herself in the Horde's eyes. If it is not a case of recovering her reputation, then perhaps there is something the worgen hold that she wants, something that will help her wipe out the living as previously planned. Or potentially, something that could restore the forsaken to their former living status.
From a writing and game development standpoint, it's not really logical that Sylvanas would out-and-out be for the mass destruction of those that she's called allies. The forsaken are a playable race, and Blizzard would not remove a playable race from the Horde faction simply because their leader decided it was time to splinter from the Horde faction and form a different alliance, or simply withdraw from the spotlight altogether. If -- and this is a big if -- Sylvanas were to revolt and try this whole "death to the living" tactic, something would have to be presented to stop her so that those people playing forsaken characters could continue playing them -- either by Sylvanas' having a change of heart, or by her death and subsequent replacement.