I'm a little late to this party, but Edge magazine had a retrospective look at the making of X-COM: UFO Defense (or, UFO: Enemy Unknown, as it was titled in Europe.)  Being the obsessive X-COM geek that I am, I'll gladly jump on any opportunity to discuss this great game.  Though Edge references the much rumored new X-COM game supposedly being developed by 2K Boston, the article concludes with a dismissive proclamation on any attempts to revitalize the series; basically, that any new X-COM is likely to fall short of the original.

X-COM has a troubled history worthy of a soap opera.  The original was a surprise success for Julian and Nick Gollop.  Wanting to capitalize on this success, publisher MicroProse set out to issue a quick sequel.  X-COM: Terror from the Deep was developed and published by MicroProse without the involvement of the Gollops shortly after the release of the original.  Critics slammed the game for simply being UFO Defense with a new setting, different weapons, larger maps, and creating a punishingly more difficult scenario (although, Terror from the Deep will always hold a special place in this gamer's heart as it was my introduction to the series.)

During this time, the Gollops had been hard at work developing their version of a sequel to UFO Defense, X-COM: Apocalypse.  Grand in scope, Apocalypse set out to expand and improve upon everything in the original.  In the end, the troubled development of the game caused a number of interesting features to be dropped from the shipped product.  Furthermore, the combination of a turn-based and real-time engine led to neither working optimally.  After Apocalypse, the last strategy-based game to bear the X-COM name was X-COM: Genesis, but the title was never seen to completion.

Others have attempted to reboot the genre of small scale, squad-based, tactical combat combined with an overarching resource management game to connect the tactical engagements to one another.  The UFO: After[...] series (Aftermath, Aftershock, and Afterlight) picked up a final failed push for another X-COM game by the original developer Mythos Games, The Dreamland Chronicles: Freedom Ridge.  Developer ALTAR Interactive sharply departed from the original X-COM games by implementing a real-time engine solely.  None of ALTAR's X-COM inspired games have been particularly compelling for me, for a variety of reasons, but there's little need to go into them here.

Another notable-yet-poor implementation of the X-COM concept was Chaos Concept's UFO: Extraterrestrials.  Excluding the laughably ridiculous title, the game attempted to recapture the essence of X-COM.  Combat was again turn-based, but so many other parts of the original X-COM had been whittle-down to nothing or outright removed that this latest attempt fell far short of a the standard set by UFO Defense.

I've written my thoughts on X-COM and what makes the game so compelling, even to this day, in many different, dark corners of the internet for as long as I can remember.  I've had a lot of time to think about, synthesize, and refine these thoughts, so I'm happy to present them once more.

There are two overarching components that make X-COM the addictive experience that it is: the degree of control and the title of "director" awarded to the player.  These two concepts create an interlocking experience for the player that hasn't been fully recaptured by any of the spiritual successors.

The degree of control is best evident in the meta-game strategy layer of X-COM.  Base management gave the player a load of interesting choices to make that were all interrelated to one another -- scientists needed to be hired to conduct research, engineers for manufacturing, and soldiers for your ground missions.  Equipment could be purchased from a general store, and their use and function varied greatly.  Interceptors were purchased and maintained by the player to shoot down detected UFOs.  Bases even needed to be properly garrisoned in the event of a ground assault from the alien menace!

I'm oversimplifying a lot of what the player can do, but the important thing about the strategy-layer in X-COM is that all of these choices are meaningful to the player and are interrelated.  Allocated scientists to research new alien technology produces better equipments for soldiers.  Engineers then use that research to build equipment for use in the field.  Equipment takes up space in your base and must be managed through a series of storage units.  Interceptors are launched from hangers to shoot down UFOs and troop transports bring your soldiers to battle.  Alien technology found in the tactical game can be researched by your scientists, and the cycle continues.

To make all of these decisions meaningful, X-COM provides a goal that focuses the player -- the need to keep the funding countries happy (since X-COM is a product of the United Nations) in order to maintain the necessary funds required to continue operations.

This degree of control over the X-COM organization provides a sense of personal stake and attachment to the success of the organization as a whole.  But the amount of control that creates this personal attachment isn't solely delegated to the strategy level -- the tactical combat part of the game continues the tradition.

One of the key reasons that X-COM offers a deep degree of control while not also becoming overwhelming for the player is the fact that the engine is turn-based.  I'm aware that turn-based is like a dirty word for those in marketing, but I stand by this statement.  X-COM's turned-based engine allows the player to make numerous tactical choices without feeling rushed or overwhelmed.  Though a real-time engine generally has a pause function, I feel that such functionality really breaks the pace of the game.  A turn-based game keeps the pace feeling natural, while the real-time game makes tactical engagements feel like they're being played out in slow motion.

In addition, the turn-based aspect of X-COM's tactical play also plays a key role in the player as "director."  I mean director in the sense of a filmmaker -- X-COM gives the player the opportunity to create original stories within the game, and the turn-based engine is a catalyst for this function.

Because the game is turn-based, the player is essentially forced to focus on one soldier at a time.  Such close attention to the movements and actions of individual soldiers creates another sense personal attachment.  This close focus also provides the nerve-racking tension of tactical battles.

Your soldiers have readied themselves for entry into a downed UFO.  You ready yourself to send in your first soldier -- will your soldier be shot by reaction fire, or will your soldier have the ability to fire off the first round?  You won't know until you send him or her into harm's way, but the key is that you will know and your attention won't be diverted at a key moment by events unfolding on the other side of the map.  You will watch this story unfold, in sequence, without interruption; a story that you, the director, initiated, shaped, and completed in one step.

And you'll create hundreds of these stories, many of them more memorable than the ending of the game.  You'll remember the time that rookie soldier successfully gunned down three aliens against all odds.  You'll remember the first time you encounter Ethereals and had your top captain mind-controlled, who then proceeded to gun down your own soldiers before even setting foot outside the transport.  You'll remember the daring escape of another soldier who dropped his or her equipment in order to carrying a bleeding comrade back to the transport for dustoff.  You might even add another personal touch by naming all of these soldiers after people you know.

This is really why I think X-COM is so compelling after so many years -- the game is really about creating a story of a rag-tag group of rookie soldiers who save the world.  You're there from the beginning, planning everything, and watching every event unfold.  Add in X-COM's randomized maps, events and enemy AI, and completely destructible environments and you've got a very unpredictable game that feels refreshing every play through.  Every game your story will be different, and this is one gamer who can't wait to write the next epic tale.