The X-Com series was one of the highlights of my time gaming on PC's back before you needed an upgrade every 4 months to stay up to date on hardware.
It was strategic, and tactical, it had lots of aliens, and above all it was HARD. And when I say hard? I don't mean 'a little challenging', I mean your first play of the game will end in complete failure. Then you learn a bit more about how the system works, and maybe you'll survive with your squad intact for the first mission. Then you get to save, and research and fund your operations, and it gets MORE challenging 'cause you've got a LOT more ground to cover.
It made it very different from most of the games on the market today that tend to hold gamer's hands. Granted, that aspect wasn't entirely unusual in the market then, but the fact that you could permanently lose agents and assets, and the challenging difficulty of the monsters all made it a seriously challenging game.
That you had research, alien aircraft takedowns, invasions to take care of all made the game complex and interesting. Now our good friends at 2K games have updated and re-done this marvelous bit of gaming history for modern systems and it's FANTASTIC. In your tutorial mission, they show you how hard the game is going to be. But there are all kinds of things that you need to learn along the way through play, just like the original.
There is ALWAYS something you're looking to get, new gear for your troops, new vehicles, new rooms for your base, new stuff to research, and never enough time or money to get it all done. The base portion of the game is all about prioritizing, while the tactical combat is all about keeping your troops alive, because there is NO magic reset button. A bad mission will mean you loose all 6 of your top people and completely cripple your game.
The only game that really compares to it is Dark Souls. Granted in Dark Souls you will die to learn new techniques and how to move through the game environment, XCom puts you in command of a team that does so. The more successful your team, the more likely they are to make it farther in the game, but the less you'll want to risk them. It's an interesting mix of cost/reward.
The game rewards patience and learning it's system, instead of the manic pace of the run and gun first person shooters that dominate the market.
It's a wonderful change in style and play for the game market, and I hope we might see something similar with the old Jagged Alliance mercenaries style game, it'd be a great step for the design team to take.