The Shattering will include the intro cinematic to Cataclysm ahead of its December launch, as well as most new content excluding the highlights.
You won’t get to play as the new races, enjoy the new zones, fly your mounts in Azeroth or enjoy the new 80+ quests and loot until Cataclysm officially releases December 7th. You can experience Eastern Kingdoms and Kalimdor torn a new one though.
The reason The Shattering is releasing now is so that players can get acquainted with the new classes made available to the existing races.
”Players will be able to find new and exciting sights to see and quests to do in the level 1-60 zones, all-new troll and gnome introductory experiences, redesigned capital cities, new race/class combinations, and more!” blurbed Blizzard in their development notes.
Looking forward to surrendering your life to World of Warcraft: Cataclysm?