Yes, this is it! The special sauce of the original is that there is so much mystery; "Enemy Unknown" is the whole idea. While I love the Firaxis reboot, and think it has better gameplay, I'm not sure it's strictly a better game. It all feels so clean and organized, and it's always clear what needs to be done. There's no sense of bureaucratic panic, wasted time on pointless tech-trees, or over-investment in a singular hail-mary plan that is completely foolhardy only in retrospect.

The discovery portion of XCOM matched with the dreadful mystery of XCOM, overlaid by unknown levels of complexity, is what makes the game feel real/alive. It makes the gamespace feel much more wide-open than it really is. When you first experience your own base under attack, the feeling is "holy shit I didn't even know this could happen and wow I did not plan appropriately for this eventuality!" In the reboot, it mostly just feels like another mission, where the layout is totally decoupled from the actual decisions you've made. And you're much more likely to have trained commandos at homebase rather than a ragtag bunch of rookies with "failed experiments" gear (eg laser tech and maybe some grenades lying around).

That's just one more example to illustrate how the original XCOM was as much about creating tension through the limitless possibilities of the unknown. Each and every decision was less like a chess game and more like a desperate gambit.