It's still a little early to say, because most of my first two hours was spent playing through the tutorial, but it appears as if X-COM: Enemy Unknown straddles the line between story-driven and randomly generated game. I may be wrong, but the missions where you have to decide between saving one place or another feel procedurally generated to me.
It's an interesting approach. Because X-COM: Enemy Unknown has a story, with voice actors and everything. I have to assume the cutscenes are triggered by the "additional objectives" (such as capturing a live alien) which can be done in any random mission.
That's my working theory, at any rate. I haven't seen too much of the plot yet. Luckily, it appears that I may be able to understand it for a change. Aliens are abducting people all over the world, and you are the commander of a secret, multinational military organization (called X-COM) that is trying to stop them. You get your funding from the national governments of the countries you protect, and you answer to an even more mysterious organization called "The Council."
I'm a little fuzzy on some of the details, like exactly how widespread and obvious all this alien activity really is, or anything about The Council's goals, identity, or mandate. Clearly, the alien threat is serious enough that all the nations of the world put aside their differences to finance and organize a joint defensive effort, but not quite serious enough that they feel the need to be publicly seen to have a response. And I'm completely baffled as to why the Council representative appears only shrouded in shadow. I'm part of the conspiracy, so who am I going to tell?
Hopefully, future revelations will clear things up, though I suspect most of the mysterious seeming-inconsistencies are merely genre conventions. It wouldn't be much of an alien-invasion story without a shadowy government conspiracy, even if it doesn't make a great deal of sense.
From a gameplay perspective, X-COM: Enemy Unknown is not yet as difficult as I feared. I've lost five soldiers so far, but no more than one per mission, so I think I must be doing adequately. I don't like losing troops, however. As weird as it sounds, I feel like I've failed them as a commander. Like, maybe if I didn't make such bad decisions, they'd be able to make it out of the missions alive.
I suppose that you can't fight a war without expecting some casualties, though often in video games you can defy the laws of reality and achieve the impossible. Sometimes, it's even expected that you do so. Whether or not my losses are small enough to endure the extended campaign remains to be seen.