SPOILER: [Lore] World of Warcraft: Cataclysm

[Winterherz]

Horde

• Garrosh Hellscream is the new Warchief and has reinforced Orgrimmar and many Horde outposts around Azeroth with dark iron. He's also reclaimed Gorehowl -- his father's weapon -- and now wields it.
  
• Thrall was captured by SI:7 (and it looks like Garrosh actually set that up), to be later rescued by the Goblins during their new player experience.
  
• Cairne Bloodhoof was poisoned by Magatha Grimtotem and is now dead.
  
• Baine Bloodhoof is now the leader of the Tauren race in Thunderbluff (those that are part of the Horde).
  
• The Forsaken went against Garrosh's wishes and released a plague on Gilneas, killing most of its inhabitants. Sylvannas is expected to be further in the dog house once Garrosh learns of this.
  
• The Horde have, thanks to Goblin ingenuity, built "The Bomb" -- a weapon of mass destruction -- and unleash it on a night elf village in Stonetalon Mountains. Garrosh, displeased with the slaughter of innocent people, kills the general who ordered the bomb to be dropped.
  
• The Forsaken have attacked and destroyed Southshore in Hillsbrad Foothills by unleashing a plague upon it. Green ooze covers the ground, many of the buildings are damaged or destroyed and a haze floats above the ground.

Alliance

• Magni Bronzebeard attempted some sort of ritual in Old Ironforge and was turned to stone, leaving Ironforge without a single ruler. While attempts are made to remove the curse from Magni, Muradin Bronzebeard, Moira Thaurassin and Falstad Wildhammer have formed a council of rulers.
• Varian Wrynn has devoted resources to upgrading many Alliance outposts on Azeroth, including Sentinel Hill.
• The worgen "curse" was actually created by a sect of druids known as the Druids of the Scythe, wielders of the Scythe of Elune and the magic it provided. They fell victim to the feral nature of the worgen and lost control of themselves -- forever transforming. They had to be locked away in the Emerald Dream, though Archmage Arugal managed to free some and in turn released the curse upon the denizens of Gilneas. The trees you see in various screenshots with the glowing blue liquid flowing from them are trees where the Druids of the Scythe were contained and put into slumber.
• The Park in Stormwind was indeed destroyed by Deathwing after he emerged in Azeroth. This is confirmed by a group of NPCs chatting before the boat ride out to Vashj'ir. Some had previously thought that an earthquake may have destroyed it.

Other

• Tiron Fordring and the Argent Dawn have taken over Hearthglen. Tirion Fordring now resides within Mardenholde Keep; his Eastern Plaguelands version at Thondroril River has been removed.
• The Alliance and Forsaken are fighting over control of Andorhal and the remaining Scourge presence there. The Alliance, lead by Thassarian, have taken control of the western portion of the town while the Forsaken, lead by Koltira Deathweaver, have taken control of the eastern portion.
• Edwin VanCleef is dead. His daughter, Vanessa VanCleef is now the leader of the Defias Brotherhood. She has unleashed a plot to bring down Stormwind after witnessing the assassination of her father some 5 years ago.
• Corporal Keeshan is the main character behind the Redridge zone progression, known as John J. Keeshan. The story revolves around the Blackrock Orcs planning an invasion of Lakeshire, and plays out much like a Rambo movie where you must recover Keeshan's bandanna, bow, knife and a locket from his lost love, as well as freeing his captured Bravo Company friends.
• Kalec is rounding up all of the blue dragonflight, as he fears the death of Malygos will be a sign of weakness and Deathwing will send out agents to assassinate the remaining dragons within the flight. This holds true as the black dragonflight has invaded Azshara to hunt down Azuregos. Kalec and Azuregos help low level Horde players to repel the invasion.
• Malfurion Stormrage is the only thing keeping Darkshore from being devoured by the Cataclysm; he's created a giant twister in the middle of the zone.
• The Twilight's Hammer are attempting to summon an Old God in Darkshore, Soggoth the Slitherer.
• Deathwing has summoned Ragnaros and agents of the Plane of Fire into Hyjal. The players go through a linear quest chain culminating in the banishment of Ragnaros back to the Plane of Fire. Involved in the chain (and fights leading up to and including Ragnaros): Ysera, Aspect of the Green Dragonflight; Malfurion Stormrage, Jarod Shadowsong, Cenarius, Goldrinn, Aviana, Aessina, Tortulla, Hamuul Runetotem, Fandral Staghelm. It's pretty damn epic.
• The Cenarion Circle and Cenarion Expedition are the reason for Desolace seeing the growth it has, and have thus named the middle of the zone the Cenarion Wildlands.
• Ysondre is the last of the Dragons of Nightmare, and although she has been saved from the nightmare, she states she can never return to the green dragonflight. Emeriss and Lethon are already dead; Ysondre helps players kill Taerar during a quest chain in Feralas.

Source​