You Can Go Home Again, Provided Home Isn't Suddenly Underwater, Subterranean, And/Or Part of An Active Volcano
Instead of inventing a new land mass with its own set of quests, objectives, and Big Bads, Blizzard opted to send its playerbase back to the original continents of Azeroth and Kalimdor immediately after the dragon Deathwing beats the snot out of them. The changes are significant enough to give long-established players a reason to re-roll while offering potential new players a chance to jump into the game at a time when even the first launch servers are going to feel relatively new.
Players who don't like leveling alts (and I'm one of them) may find reason to do so in Cataclysm. The twin pulls of nostalgia and curiosity are powerful, particularly when the expansion opens areas of the game that we've literally been closed off since WoW's launch in 2004. Even in zones that weren't particularly damaged, there's a sense that time has passed. Projects have been finished, old conflicts resolved (often thanks to pyroclastic lava flows) and new battles have erupted.
Now that we've addressed the lore, let's clear the usual topics off the table. Cataclysm includes: new quests, storylines, zones, spells, two additional races, artwork, achievements, recipes, items, mounts, jokes, bosses, and events. If we neglected to mention something that falls under the category of "Things One Normally Finds In An MMO Expansion," you can assume it's also present.
After six years of fighting between Southshore and Taure...err, Tarren Mill, the Horde won this one. On the plus side, the real estate deals here are even better than you'd find in Florida.