A few months ago, I had a conversation with my dad about my Warhammer 40,000 army. It was a thought-provoking experience. Now, Dad is the person who taught me everything I know about wargaming, and my oldest playmate, so it was a lot of fun to talk with him about the use of my orks on the battlefield. But what was thought-provoking was the fact that we could have the conversation at all.
You see, Dad doesn't play 40K.
He doesn't know the rules, either. Has barely ever even asked about them. This is the thought that our conversation provoked: what is the quality of the game that allows Dad and I to have an intelligent conversation about it when only one of us has made a study of the rules?
My answer is this: 40K is a tactical game.
"Tactical" is a buzzword when discussing games, particularly nerdy games that depict violence. "Tactical" the buzzword often means nothing more than "has choices associated with it." I don't find this to be a very helpful definition, since all games present the player with choices to make, so this definition usually ends up focusing on what the speaker things are the important choices. Compare Dawn of War 2 to StarCraft. Both are real-time strategy computer games. In the latter, one of the high-level choices players must make as they play the game is how much of their time and energy to devote to the holy triangle of engaging the opponent's forces in battle (offense), building their own forces (economy), and defending their means of production from the opponent (defense). By and large each of these three tasks is independent from the others, and finding the right balance among the three is one of the most important challenges of the game. Dawn of War 2 does not have this triad of decisions (a triad which has become one of the hallmarks of RTS games), focusing instead on decisions such as when to preserve and when to sacrifice troops, and which territory must be taken and which can be ignored (or abandoned). Because Dawn of War 2 does not present the player with the offense-economy-defense triad of choices, it might be (and has been) criticized as "not tactical."
The U.S. Army Field Manual, wikipedia helpfully informs me, defines tactics as "The employment of units in combat. It includes the ordered arrangement and maneuver of units in relation to each other, the terrain, and the enemy in order to translate potential combat power into victorious battles and engagements." With this as our understanding of "tactics," then, I propose the following definition of a tactical game: A game is tactical to the extent that it admits of analogies between tactics that can successfully be used within the context of the game and tactics that have been or are successfully used by real-world combatants.
One of the obvious implications of this definition is that you can learn about the tactics of a tactical game by learning about the tactics of its real-world analogue(s). In fact, if that analogue is a popular one, by learning about it, you might be learning about multiple tactical games at once, before you even know what they are. One of the less obvious implications is that learning about real-world analogues always teaches you at least something about all tactical games, because all successful real-world tactics have certain things in common. This is the quality that 40K has that allowed Dad and I to discuss the game even though Dad knows nothing about "the game" in the sense of sequence of play, how to read a unit's statline, etc.
You might think that a "tactical" game in this sense is boring, because you can learn about the game without actually playing the game. To some extent this is true; a large part of the appeal of a tactical game is in attempting to put one's tactical ideas into practice - the fun is in the application of knowledge to form a plan and execute it, not in gaining the knowledge in the first place.
In a sci-fi context, though, there actually is an element of discovery, because things don't necessarily look like what they are. For instance, consider my orks. Although they are large and green and armed with automatic weapons, they are essentially Napoleonic infantry. They function to best effect when deployed in contiguous battle lines with mutual support. The advance of orky infantry is dependent in large part upon coordinating simultaneous impact with the enemy, in picking a time and place to advance that neither leaves one exposed for too long nor unduly disrupts the formation, and in prosecuting the attack once begun with resolution. All things that would have been familiar to a commander of infantry in the early nineteenth century, I imagine.
Similarly, my ork warbikes function essentially as cavalry - they are highly mobile, but lack staying power, and flounder against a bold defense by infantry unless they have near numerical parity. Warbikes are not to be thrown against the front of an enemy formation unless they have every advantage, and sometimes not even then. And yet my warbike-mounted nobz (bigger, nastier orks on warbikes) function not as cavalry but more like a tank - unlike cavalry, they combine high mobility with high durability and high combat power. Regular warbikers go where the enemy is not. Nob bikers can force the enemy to be where they are not.
Part of the fun here is just the incongruity between what a unit looks like and what it is. Green-skinned aliens on motorcycles don't necessarily look like Frenchmen on horses, but they function the same way. It's not obvious that if you just increase the size of the greenskin he and his motorcycle buddies morph into a single armored fighting vehicle, but there you go. But another part of the fun is in figuring out why a unit is what it is. Nob bikers are "tanks" because they have the speed, durability, and combat power to muscle the enemy aside. But they aren't actually a single tank; they're multiple big orks on Harleys. The mechanism is different but the result is the same, so the two units can be employed in the same way.
The relationship is a little more complex than that, of course; nob bikers aren't tank-like in every respect (they lack the ability to project combat power at a distance, for instance), but if you patch together enough analogies from real-world tactics you can come up with a sound doctrine for their use. Figuring out the proper patchwork of analogies is part of the discovery fun, and then you get the fun of applying that patchwork to devise and execute a plan.
Not every game admits of this kind of analogizing. Many first-person shooters, for instance, bear no resemblance to any real-world tactics. The only way to learn about the tactics of Quake is, well, to play Quake, or other games in the "twitch shooter" genre. No amount of clever analogies will change that, and all the knowledge in the world about close-quarters battle will be of basically zero use. The only way to learn about the tactics of Final Fantasy Tactics is to play that game, or other such "tactics" games. No analogies can be drawn to real world combat of any time period. These games offer lots of interesting choices and skills to master, but they aren't "tactical."