What makes a system's combat system tactical (or really any part of the system)? In short, choices and options make a system tactical, where you have to make decisions on what you do.
Well that was simple. Shortest blog post every by me. I'm done.
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No, that's not really the end of my thoughts.
In my experience, the five W's (Who, What, When, Where, and Why) can provide the factors that make combat interesting - the more of those questions you get to ask in a combat, the more interesting the combat will be for me.
To discuss in more depth, and totally out of order:
Where - Where is probably the most basic of the tactical choices. Where do you stand, do you get cover, do you move to melee, do you stand beside the fighter. This is something that the gridded (i.e., played on a map with a grip) and war games (such as Warhammer) do extremely well. You can even keep it vague, such as zones from Fate, the positioning system from Warhammer Fantasy Roleplay 3rd Edition (which if I remember correctly, as a close, distant, really far rating system) and each has its pluses and minuses, balancing speed of combat and number of options given.
Personally, I think the best balance becomes one of "zones" where you can divide the space up into logical chunks.
What (and How) - What you do is the other key choice. If there is only one "right" action to take, no matter what the circumstances are, then that's tactically boring. A lot of systems, such as new World of Darkness system, use this because the play is not supposed to be about the tactical system, I read someone describing the system as a "murder system" because it isn't tactical play per se, but about murdering someone. This is something that 4th Edition D&D did extremely well - giving lots of options to all of the classes for what they could do with each action - be it an at will, an encounter, or a daily power. Personally, for me, it made the fighter systemically interesting to play, because I had options beyond "roll basic attack to act". Fate, my current game that I'm running, only gives a few basic options of Attack, Defend, Maneuver, and Move; but maneuver covers a wide range of activities so I'm not hugely concerned with it (which reminds me, I need to do more with maneuvers in my next Dresden Game).
The How is just a variable to the what you do - all out attack or cautious attack. Not much else to say on that factor.
When - When as a tactical choice was introduced to me, in depth, with Exalted 2nd Edition and the Battle Wheel. Many of the key choices in that game were not positioning on a map (because distances and locations were generally easy to change), but the position on the battle wheel/initiative chart. I've talked about initiative a little bit ago, so I won't go too far into the weeds with this one.
Who - Who is an interesting one. It can be "who attacks" such as with war gaming, and deciding who needs to do what, when (oh wait that ties into the others). It can also be Who is attacked - do you kill the mage first, or the fighter, wipe out the mooks, or just go after the boss? This requires there to be options on who you attack - which is why I enjoy minion rules, because I don't want every major fight to be a single major BBEG against the party, instead I want there to be multiple opponents and options on the table to provide choices for the players to decide on.
Why - Why is a hard one for me to look at within the tactical combat. Some systems, such as Smallville, try to force the issue by removing standard attributes, but replacing with motivating figures. Anyone out there have thoughts on a good "why" tactical system?