None of the information below has been officialy announced by Blizzard, this is only a compilation of information gathered from reliable sources. Just keep in mind that it takes a LOT to make me post something on the front page and I'm definitely not speculating here.
Other sites started it, I'll try to finish it. Think twice before you read this post, we have a lot of spoilers here.
World of Warcraft - Cataclysm
World of Warcraft: Cataclysm is the next expansion for World of Warcraft. Dark and forgotten threats that have long stayed out of sight have finally arrived on our shores, unleashing a cataclysmic event upon Azeroth and now preparing more nefarius plans to take it back.
The expansion doesn't have any new class. Instead, Blizzard will offer more race and classes combinations to players, some of them have been datamined on the 3.2.2 Test Realms.
* Human Hunter
* Orc Mage
* Night Elf Mage
* Dwarf Mage
* Blood Elf Warrior
* Dwarf Shaman
* Undead Hunter
* Tauren Paladin
* Tauren Priest
* Gnome Priest
* Troll Druid
The level cap in the next expansion will be slightly lower than expected this time around; 85.
This suggests Blizzard wants more room for expansions before hitting the level 100 cap. With less leveling to do (along with the revamped leveling listed below likely to increase the speed and ease of leveling), leveling a new race remains appealing for new and existing players alike.
The events of the cataclysm has caused two new races to seek new allies. The Goblins for the Horde and the Worgen for the Alliance.
The Goblins have found themselves washed upon an island off the coast of The Barrens, their islands devastated by the cataclysm. As they strive to recover, they come across an Orc captured on an Alliance ship and rescue him. The Orc turns out to be none other than Thrall himself, and in return offers the Goblins a place within the Horde. These events are carried out in a quest line, allowing players to experience first-hand the reasons behind the Goblins' shift from neutrality.
It is unclear if the old Goblin locations such as Kezan and Undermine will be included in Cataclysm with the revelation of the Goblins' displacement.
The cataclysm has cracked open the Greymane Wall, finally revealing what has happened to the kingdom of Gilneas and its citizens. The Worgen have ventured forth from Gilneas and sought help from the Alliance, deciding to join them to combat the new threats of Cataclysm.
Gilneas will make extensive use of the phasing system (much like the Death Knight starting area), to show what happened while the kingdom was cut off from the rest of the world and lead up to present day.
Last but not least, Worgens will have two forms and will be able to morph between a human form and a worgen one. Players should be able to pick both appearances when they create their character.
Cataclysm will be the first expansion not to introduce a new continent, instead making use of previously unreleased zones and revamping existing ones.
Classic Azeroth Revamp
A cataclysmic event caused by Deathwing and Azshara will change the face of Azeroth as we know it. Most of the new content for Cataclysm will take the form of a revamped Azeroth, taking advantage of newer additions to WoW such as phasing and daily quests. Most of the quests and mobs in the classic zones will also be redesigned to make leveling less painful. Some zones will change drastically to fit this, e.g.,
* The Barrens will be split into two separate zones of two different level bands.
* Azshara will become a low level (~10-20) zone.
Flying in Azeroth
Part of the redesign of the Eastern Kingdoms and Kalimdor is the introduction of flying to the two continents, allowing access to many new areas and quicker travelling across the large continents.
Unreleased Zones & Dungeons
With the addition of flying mounts to Kalimdor and the Eastern Kingdoms and the redesign of many zones, most of the previously unreachable or incomplete zones will now be made available to players. This is where most of the new content from 80 to 85 will take place. Some of these are,
* Hyjal (present)
* Gilneas - The Worgen starting Zone.
* Uldum (Most likely as a dungeon)
Classic Dungeon Revamp
Redesigning Onyxia's Lair in Patch 3.2.2 was just the first step. As most of the leveling will take place in revamped areas of Azeroth, so too will the dungeons, allowing players to use them to level from 80 to 85.
As far as I know, Ragnaros will also be back in this expansion.
Deathwing the Destroyer / Neltharion the Earth-Warder will play a major role in Cataclysm. Having been driven mad by Old God whisperings and turning against his own kind only to be fail in his attempts, Deathwing has sunk into the shadows. While the Horde and the Alliance were busy fighting back the Burning Legion only to then be beset with the Scourge, he has been lurking and moving things into place. After Lady Sinestra's failed attempts to create a Twilight Dragonflight, he again retreated to Grim Batol and succeeded where she failed. With his results seen in the Obsidian Sanctum, he has finally created the supreme Dragonflight he sought and plans to unleash it upon the world. But what of the Old Gods' sway over him?
Queen Azshara will also play a major part in Cataclysm. Unknown to many mortal races, long forgotten by others, and believe to be dead by her own kind, she has not been dormant in depths of the Maelstrom. Having those around her transformed by the Old Gods into Naga after her failure in the War of the Ancients, she has become far more powerful and a greater threat than she once was. Not content with just Nazjatar and the depths of the sea, Azshara seeks to reclaim power and reign once again. With the true plan behind Lady Vashj's support of Illidan remaining a mystery we do not know what hand Azshara has yet played. The big question remains, is she now serving her "saviors", the Old Gods?
I'm still not sure who is the true end boss of Cataclysm is, but the cataclysm appears to have been caused by attempts to incantation to summon extremely powerful beings using an ancient incantation by Deathwing and Azshara. They're both very powerful, but the cataclysm itself suggests something more powerful is behind it, perhaps their shared past of Old God influence?