Why is it all capitalized? That's what I want to know. XCOM isn't an acronym for something--presumably it stands for Extraterrestrial Combat (or Command?)--so shouldn't it be XCom? I guess that doesn't look as good on the posters. Maybe they should add an exclamation point. (Or a dash, according to the purists. Luckily, having never played the original, I'm not really interested in purity.)
There aren't a lot of games where I finish them and immediately start a new session. Mass Effect 2 was probably the last example--I did two straight playthroughs, and possibly started a third, just because the basic mechanics were so solid and enjoyable. XCOM might be just as catchy, even though I didn't expect it to be. Here are three things that surprised me the first time through:
I didn't think I'd get so attached to my squad. People talk about doing this in the old X-COM, being genuinely upset when a soldier bit the dust, and I just figured those people were crazy. But about half-way through the game, letting Col. Zahara "Werewolf" Mabuza die just stopped being acceptable. The nicknames must have a lot to do with it. I knew every nickname on my squad, especially the ones that got funnier as they got more panic-prone ("Padre," indeed).
XCOM gets a lot of mileage out of only a few maps. I think I saw in an interview that there's only 30 or so maps in XCOM, which is not a lot considering the hundreds of encounters in a typical game. Partly, the maps are just well-designed: just starting out in a different space and direction is enough to make many of the UFO capture maps completely disorienting. But they're also partially-randomized, meaning that you never entirely develop a single cover strategy for each map. Add in the day/night filters, and it feels like a lot more content than it actually is.
Everything is short. Six soldiers means that you're doing with a turn in roughly 60 seconds. A mission in XCOM takes, at most, 30 minutes. Between missions, you pick your research tasks and your engineering projects and then you hit the big "GO FAST" button in Mission Control and see how far you get before the next invasion. Sometimes a movie plays--they're all skippable, as are all the little interstitial animations (launching a fighter, landing the SkyRanger, etc). Everything in the game is made with the understanding that you Should Not Wait, a convenient side effect of which is that it's compulsively playable.
It's not a particularly profound game. It's not even particularly well-made--bugs pop up all over. Even with the tutorial, I restarted the game twice trying to figure out how to keep everything balanced, which is pretty hardcore. But it's so consistently fun that those problems don't halt the experience. I never really got the Halo philosophy of "30 seconds of fun" because I find Halo to be a boring, frat-boy knockoff of better shooters, but XCOM pulls it off.