Many blockbuster, high-budget games would like to be earthshattering. World of Warcraft: Cataclysm happens to take the phrase literally.
In case you've been in a coma: After spending a few games asleep within the earth, Big bad dragon Deathwing wakes up and disrupts the forces binding the earth together, breaking the world. It's tricky reviewing a game like Cataclysm because it's intended to be a different thing to many different audiences. If you've never played WoW before, if you used to play WoW but left the game years ago, or if you're a current player you'll likely feel very differently about the third expansion pack to Blizzard's mega-MMO. So let's start off with the commonality: Cataclysm tears apart ten years of WoW development and puts it back together better than it's ever been.
The bulk of Cataclysm coverage has been (rightly) focused on what the WoW team has done to the old world of Azeroth. Recognizing that classic WoW's "go kill 10 kobolds, now go kill 10 more kobolds" quest content was actually pretty terrible, at least in comparison to the leveling content and quests introduced in Burning Crusade and Wrath of the Lich King, Blizzard has taken a heavy hatchet to its previous work.
While some quests and storylines remain mostly unchanged, the majority of content is new and implemented with a graceful technique that simply wasn't there when the game launched six years ago. What had been a simple fetch quest now involves the player watching an angry debate between three rival clans and the hunting down of a traitor within one of them. It's much more engaging than old WoW had been, with a larger emphasis on progressing through a story instead of a static, unchanging world, and is a sorely needed change to painfully archaic content.
Less has been written about changes to the game's systems, and with good reason; that discussion is one that matters most to current WoW players. If you're the type of person who doesn't know your hit from your crit, then the specifics don't matter: What does matter is that Blizzard has taken a game wobbling under the weight of an ever-growing Gordian knot of stats, skills and class bonuses and cut that knot in two.
If you're new to WoW or returning after a long absence, you'll find the newbie experience significantly improved. Not only are the quests more engaging, but the mechanics and systems have been almost wholly reworked, and the game provides players with far more useful information than it ever did before. Every time you level up, the game informs you what new skills (if any) you have unlocked for training, and the tooltips explain how an ability is used instead of dispassionately informing a player of the exact numerical properties of the attack.
In old WoW, a player would hit level 10 and unlock their various talent trees that determined their character's strengths. For someone new to the game, the only information they were given was what was listed on the tooltip, making it difficult to know good builds without outside aid. In Cataclysm, hitting level 10 still unlocks your talent trees, but the way in which it's presented makes it much easier to understand. The strengths and weaknesses of each skill tree are laid out from the beginning, and choosing one will grant you a powerful and iconic special ability right away. It feels more like choosing a subclass in a game like KotOR than simply putting points into a tree.
This change in focus accompanies a clear attempt to make every class play in a more interesting and engaging fashion than "hit, wait for random-chance ability to activate" or "spam your damage spell over and over." While the core combat is still very much WoW, there's an increased focus on reactive abilities that feed off of one another, encouraging players to react and change things up instead of going through a standard by-the-numbers rotation.
Other than the general remaking of the world, it's hard to quantify any single thing that makes for an improved questing experience whether you're level 8 or level 80. Instead, it's a bunch of little things that all add up. Quest hubs lead neatly to other quest hubs with breadcrumb trails, important NPCs are highlighted in a quest sidebar that tells you things you need to know (e.g., "this guy is on fire, drag him into water to weaken him"), and if you ever get lost, the hero's message board in each major city will point you in the right direction.