Just north of the Howling Fjord are the mountainous Grizzly Hills. For most Alliance players, your quests here begin at Amberpine Lodge, where you are tasked with gathering food and supplies for the war effort.
Horde start off in Conquest Hold.
After the introductory quests, the first the three major chains in the zone begins. You are sent to provide aid to a small camp within the forest.
You are tasked with subduing some of the hostile ice trolls that have begun gathering to the south. You capture one alive, and bring it back to the camp. Probably against protocol, you talk with him, and he becomes a questgiver named Drakuru, promising to show you the secrets of the mysterious Drak'Tharon Keep, a large Troll fortress that blocks the way to Zul'Drak in the north.
After a series of quests where you travel around the Grizzly Hills and kill various groups of trolls, Drakuru reveals through a spirit link with you, that you have done well, and tells you that your final task is to enter Drak'Tharon Keep, make your way to the top, and use an item at the fire on top of the dungeon.
Meanwhile, a questgiver in Amberpine sends you to the nearby village of Silvermoon, hoping to strike an arrangement with the settlers there to provide support, if needed.
The settlers agree to help, on the condition that you clear all of the mysterious plant known as wolfsbane from the town's borders, which, of course you do and find in no way suspicious.
Once again, I've been had.
Naturally, since this is the expansion of you unwillingly aiding the enemy, Silverbrook is, in reality, a camp of hostile worgen that you have essentially set free. You are only narrowly able to escape, after finding out the truth from a caged prisoner in one of their lodges.
You investigate further, and found out that Arugal, who was the ruler of Shadowfang Keep prior to Cataclysm, and the dark wizard responsible for summoning the worgen to Azeroth, has been resurrected and is located in a keep off the coast of the Grizzly Hills, churning out more evil worgen to do his bidding. You head out there, and take care of him.
Another quest chain has you dealing with hostile Iron Dwarves, who have begun to pour into the Grizzly Hills and as far south as the Howling Fjord from the Storm Peaks, which lie to the north.
You are able to cripple their efforts to restore their ancient stronghold of Thor Modan, and later, turn your attention to their mountaintop base of Dun Argol. Here, they are building a massive army of iron golems, which they plan to use in their effort to defend their master, a giant titan named Loken, in his kingdom of Ulduar.
Finally, you explore the roots of a failed attempt at a World Tree, planted by druids hundreds of years ago. There is some kind of corruption deep in the roots which made it unable to grow, and is driving the furbolg inhabitants of the tree insane.