Which part? Where she got left behind, or where she murdered eight billion people? —Jim Raynor, to Matt Horner
By the beginning of Wings of Liberty, Raynor is living next door to alcoholism and one block over from despair.
James Raynor was one of the main characters of Starcraft and he's the central character of Starcraft 2. In the original game, Raynor started out as a peacekeeper on a colony world and was forced to ally himself with a renegade organization led by Arcturus Mengsk to rescue his people from the onslaught of the Zerg. Mengsk turned out to be what you might call a bad seed; the would-be liberator deliberately exposed the world of Tarsonis to the Zerg knowing it meant the death of several billion people. Not satisfied with such a general act of douchebaggery, Mengsk also deliberately abandoned his partner, Sarah Kerrigan—a psychic Terran operative, and a woman with whom Raynor was falling in love.
Later in the game, Raynor—having begun to come to terms with the fact that Kerrigan was dead—discovers that she isn't. The Zerg Overmind, having found Kerrigan on Tarsonis and recognized her latent potential, has infested her and turned her into the (literal) Queen Bitch of the Universe. The rest of Starcraft and its expansion, Brood War, boils down to The Galaxy vs. Sarah Kerrigan (aka the Queen of Blades). Kerrigan wins.
SC2 picks up four years after Brood War but from the look of Raynor, you might think it'd been a decade. When Raynor abandoned Mengsk in the wake of Kerrigan's death, he swore to expose the tyrant for who and what he was, but four years later, his success at doing so has been minimal. The game kicks off with Raynor returning to Mar Sara to investigate suspicious Dominion activity and ends on the volcanic Zerg homeworld of Char. In between, you'll dodge natural disasters, steal cargo, capture military prototypes, and experience the Protoss equivalent of a mind meld*. This last is a nifty plot twist that lets players experience the game from the Protoss' perspective, with enough missions to get a feel for the race.
In between missions, players are free to wander the Hyperion, where most of the game takes place. While access is initially restricted to certain areas, the game eventually opens multiple distinct areas on the ship, each with its own NPCs and functions. As the campaign progresses, you'll be offered the chance to research Protoss and Zerg technology, upgrade units, and hire mercenary bands to improve your chances in the field.
The RTS genre has always been plagued by the difficulty of designing individual missions that feel unique when each game follows exactly the same path of building up a base, managing resources, and building enough of Unit X to succeed at Mission Y. SC2 tackles this from several angles, including unique mission challenges, unit flexibility, and a wide variety of mission types. Because the single-player campaign is entirely separate from the multiplayer game, there are a number of single-player units to experiment with. This also allows every unit from the original game to return, which makes for much preferred continuity.
Hydralisks: Not friendly, not domesticated, and definitely not vegetarian.
If you've got nostalgic memories of hosing Zerg with firebats, you can opt to buy their upgrades back on the Hyperion, or alternately use the Factory-built Hellion ATVs which trade staying power and bunker capability for speed, base damage, and maneuverability. (Fully upgraded, the Mercenary Firebats can take a massive pounding when backed up by Medics - Ed).
Available difficulty levels include Casual, Normal, Hard, and Brutal; difficulty level can be adjusted on a mission-by-mission basis. We started on Normal, just to get the hang of things, then graduated to Hard for the remainder of the game. If you enjoy near-frantic pacing, some of the last stand style missions are great fun to play on higher difficulty. All of the missions include achievements, all of which are split between the Normal and Hard difficulty levels.
*Pro Tip: Game devs and sci-fi writers, please stop using the word 'universe' as a substitute for 'galaxy.' Describing impending doom in ludicrous terms is annoying; it's the functional equivalent of claiming that a hurricane or tsunami threaten to destroy the entire planet. Describing the Xel'Naga / Reapers / Goa'uld / Whatever as threatening the universe doesn't build tension, it just makes the speaker sound overwrought and uninformed.