If you've been proud of yourself for resisting, you might want to look away, because there are many reasons to start playing again or start up for the first time. The expansion has brought with it a number of improvements and boosts to the game, some obvious and some not. If you've been considering coming back or trying the game out, there's never been a better occasion than right this moment -- whatever your reason for leaving might have been.
I am sick to death of the old content
Playing World of Warcraft for enough time meant seeing everything the game had to offer -- several times over. At lower levels, you could conceivably avoid certain zones, but as you got higher in level, it was more or less a necessity that you got funneled into the same areas, and as a result, every character heading up the leveling ladder would wind up in the same darned zones, killing the same darn zombies or bugs or whatever.
It's neat the first time. It's not neat the second time, and it's boring enough to make you gouge your eyes out by the seventh time.
It's not possible to say that you'll never get bored of the new world, but there has been a massive overhaul in the entire experience in the old world. High-end quests have been brought down to the base of the game, meaning that lessons learned from six years of development are now present from the ground up. The handful of old quests that are still in place are the ones that weren't quite so horrible to start with, and the new quests and landscapes play to WoW's strengths. It's a whole new park with a very familiar system.
I really want a new class to play
People have said for years that the game's classes functionally had three separate sub-classes, but it was a distinction the game has never really enforced. As time went on, talent trees got more and more bloated, and there were certain specs in which players were almost expected to do something counter-intuitive like not picking up the ability at the bottom of the talent tree. You wound up with weirdness -- abilities that make perfect sense as design priorities at the top of the tree keeping early leveling a chore. Fury Warriors, for instance, either had to level with a two-hander and play like a weak version of an Arms Warrior, or they had to level as a different spec and then change entirely once Titan's Grip became available.
The new system is a universal improvement over the old one. In short, it acknowledges the three sub-classes within each larger class. At level 10, you immediately choose one of the three trees, and that tree is where you will be putting your points until you have 31 points in that tree. Talent points are awarded every other level, rather than one per level, and as a result, the trees have been massively streamlined.
Choosing a spec also gives you a variety of abilities related to that spec right away, specialized traits and eventual bonuses based upon that spec's playstyle. An Enhancement Shaman gets Dual Wield and Lava Lash, along with a chance to recover mana from melee hits and a boost to spell damage based on attack power. That happens the moment you choose to spec down that tree, giving you the spec's signature abilities out of the gate instead of when you've finished the leveling game.
The classes are structured differently all around. Signature abilities have stuck around, but the bottom line is that each class has been redesigned from the bottom up to play as intended from level 1 onward. If you're used to playing a Retribution Paladin at level 80, you might not see a huge change... but start one from the beginning, and it's a whole new ball game.
I don't like being so limited in my choices of race for a given class
If you dislike the tree-hugging narcissism of the Night Elves, you've been kind of stuck playing a Druid on the Alliance side for a long while. Playing a Paladin in the Horde meant you were playing a Blood Elf. And for some reason, Nathanos Blightcaller never bothered to teach any other Undead how to hunt things, despite hunting's remaining a fairly standard practice among human beings (see also: beginning of species to present).
Reworking the classes from the ground up spread to expanding class choices laterally, allowing many races to pick up skillsets that were previously unavailable. Humans and Undead can become Hunters; Trolls can get in on the joy of shapeshifting as Druids; and for the first time since the game was in beta, Dwarves can become Mages. The only race not to acquire any new classes was the Draenei, with every other race gaining at least one new class option.
Furthermore, with the expansion's addition of Goblins and Worgen, options spread even further. If Trolls, Orcs, and Tauren don't appeal to you, perhaps a Goblin Shaman is more your speed. Worgen add another Druid race to the Alliance's arsenal, as well as offering a more feral choice for Rogues or Warriors. The dozen races make the game nothing if not an exercise in heterogeneity.
The endgame of WoW is probably going to change around the same time that the Earth is swallowed by the sun, but there are some definite improvements baked into the expansion. The biggest one is probably the new secondary profession of Archaeology, which allows players to leave the slaying of dragons behind in favor of traveling the world in search of ancient relics. While the discarded Path of the Titans concept was the original tie-in for Archaeology, it's been reworked to be a fun and more casual activity for players to enjoy.
The system that used to be badges for dungeons and honor for battlegrounds has been reworked as well. Both systems are now point-based, with the fundamental conversion being that raiding and arena matches are the fastest way to get the top-tier equipment. Daily heroics and rated battlegrounds will both award players with the same currency, allowing players who aren't fond of the playstyle to still progress -- albeit not at the pace of the more devoted communities.
And if neither of those floats your boat, there's strength in numbers. Guild perks allow guilds to level up and gain newfound abilities based on members banding together and advancing. This includes a variety of special rewards. So if you won't do it for yourself, maybe you'll be inspired by thinking of the children.
I've never even tried the game
You're in the same boat as everyone who left six months ago and vowed never to return.
If you've been playing WoW for years but finally got bored to death, then certain things are going to mean more to you than to someone who has never picked up the game before. After all, a new player isn't going to appreciate the changes to the Plaguelands following the events in Northrend, or the splitting lava that ruptured the Barrens. But both of you will be equally capable of appreciating taking flight within Azeroth, experiencing an unfamiliar world, and enjoying the new opportunities present in the expansion.
Starting new is the best way to experience Cataclysm. It puts you down on the ground level and lets you enjoy everything that has changed for the better -- and nearly everything has. Vehicles, storylines, and surreal landscapes will be new and bracing for you. It strips away any preconceived notions, any long-held resentment about what the game ought to be -- it's a chance to enjoy the game just for what it is.
And it's a fun themepark romp. It pretends to be nothing more or less. So go ahead, submit to the call, scratch the old itch, and get back into the game. And when you find yourself resubscribing for another three months, you can feel free to blame me.