Through completing the side objectives in most missions, you can gain either Protoss or Zerg research points. These are added to a separate tech tree which gives you a choice of upgrade at each 5-point level and the research can add new units or change the way your buildings work, permanently altering how you play. The only drawback is that you can only choose one research project at each level, with the other option lost forever, but that makes for some compelling decisions to be made.
Playing the 26+ missions in the StarCraft II campaign never feels repetitive or old hat. Yes, there are common base-building tasks, "defend the hill for X amount of time" missions and squad-based tactics missions through smaller locales, but each feels like a unique experience. Perhaps that is because the missions are closely tied to the characters that propose them to Raynor. Gabriel Tosh is a rogue Ghost who now refers to himself and his followers as Spectres, elite cloaking units with unique abilities. If Tosh wants to free prisoners on New Folsom, then it follows that his Spectre unit will figure prominently in the mission. New units are added to your arsenal in missions that let them shine, such as the Diamondback being salvaged in a mission that has you chasing down fast-moving Dominion hover-trains.
Even though StarCraft II is fairly derivative of recent sci-fi television series (there are notes of Firefly and Battlestar Galactica), it feels original for some reason. It could be the strong Southern American influence of the Terran race; most every character has a strong accent or at least a twang and the soundtrack is more Southern Rock than sweeping orchestra. The cinematic story definitely has an Us vs. Them theme which feels very Southern.
That's not to say that the cutscenes are perfect. The dialogue can feel forced at times, and most of the representations of characters reside firmly in the uncanny valley. When a character's expression changes only once or twice in a 3 minute conversation, it just doesn't feel right. The leadup to the conclusion of StarCraft II is quite impressive, as Raynor is caught up in universe-altering events, but, when it was all over, I felt that the last scene didn't give me enough. I was left asking, "What happened? Did it work? How long do I have to wait for Heart of the Swarm?!?" Unfortunately, there's no word on when Blizzard will release the next installment. My guess is 2015.
One might argue that the single player campaign is a mere preamble to the multiplayer battles of StarCraft II, where your mettle is tested against the multitudes. I disagree. Despite the pressures of recreating the success of the multiplayer masterpiece of the first StarCraft, Blizzard obviously didn't put all of their eggs into perfecting just that portion of the game. The essence of StarCraft II is the saga of Raynor against the Zerg-infested Kerrigan and the struggle of freedom versus oppression. These themes are far from clear-cut, however; is Raynor's love/hatred of Kerrigan more important than the human race? Is freedom important when humanity is threatened by the Zerg?
As in any piece of interactive art, it's up to you to decide. The genius of StarCraft II is that these decisions rest in the very framework of how the game is played. By playing the missions, you embody Jim Raynor and decide where to place your resources and how to accomplish your goals. StarCraft II is not an open-ended experience, it is, in fact, fairly railroaded as modern games go, but it never feels like your decisions are meaningless. How you play, how effective you are as a battlefield general, matters.
Bottom Line: StarCraft II is a wonderful game, both as a story-telling experience and a strategically deep and tactically challenging game. The single-player campaign is deeply satisfying to complete.
Recommendation: If you have a games-ready PC, buy this game. If you don't, buy a decent PC and then buy StarCraft II.
Greg Tito would like to be Egon Stetmann's friend.