However reaction moves are not without flaws. The extra interactions, whilst providing many decision points, can slow the game down dramatically. The "reactions" can promote passive, "camping" gameplay and cut down on maneuver. The extra rules add complexity. (Tomorrow's War has situations where units are reacting to units who are reacting to other units... yes it hurts my head thinking about it too....)
Let's define some reaction types first. The parameters of a reaction vary a lot, from simple "overwatch" mechanics in 40K to full blown unlimited reactions (Infinity) where what you do in your opponent's turn is often more important than what you do in your own.
How often can you react?
Very Limited (many games)
Each unit gets a single reaction, and then that's it. A token is placed beside the unit to show it cannot react again.
Limited Common Pool (Lords & Servants)
Have activations in a single pool, and at the start of the turn you divide them up to use in either your own or your enemies' active phase. I.e. you have 12 activations, and you plan to spend 7 of them in your active turn, and 5 reacting to your enemy in his. This could also include common mechanics like "passing" your turn to acquire an overwatch token, which kinda falls into the "very limited" box.
Diminishing (Tomorrow's War)
Each unit can react multiple times, but it becomes less effective each time. You track the # of reactions (say with a d6) and the amount of the reactions reduces firing or melee dice or similar - or decreases your chance of a reaction. I.e. lose a firepower dice each reaction. Thus reactions are limited by initial firepower/combat stats.
You can react with every unit that can see any single enemy unit acting in LoS. As many times as needed.
What triggers a reaction?
Actions only directed at the reacting unit. Sometimes a unit can only react if the shooting is directed at it, or the charge is made in its direction.
Movement/Shooting/Both (many games). Sometimes any shooting triggers a reaction. Other times units can only react to moving units. Often either moving or shooting triggers a reaction.
Anything (Infinity). If a model scratches its bottom, you can react to it.
How Easily Can You React?
Automatically. If the unit triggers a reaction, you can carry it out.
Pass a dice roll. You need to pass a target number on a dice roll to react. I.e. 2D6s vs Leadership.
Opposed Roll. (Infinity, Tomorrow's War) You need to beat your opponent's roll, and a target number as well.
What range can you react to?
Set proximity (many games). I.e. any trigger within a set range - say 12" - allows a reaction.
Unlimited/LoS (Infinity). I.e. reactions are triggered by actions in line of sight, with theoretically unlimited range.
What Order do you React In?
Single vs Single - sequentially (many games). A single unit acts, it resolves any reactions by enemies one-by-one. I.e. Unit A activates, it resolves any reactions by Unit X, then any reactions by Unit Y, then after that, Unit Z.
Multiples vs single - simultaneously (Infinity). If a single unit acts, multiple units can target it and attack it simultaneously. Unit A acts, and must simultaneously roll to beat Units X Y and Z.
How severe is the reaction?
Weakened (Infinity). Attributes are significantly restricted i.e. rate of fire is reduced from 3-4 shots to 1 shot only.
Diminishing (Tomorrow's War). Reactions are initially full strength, then gradually decrease in potency.
Full Power (Lords & Servants). A reaction shot is every bit as good and potent as an ordinary shot.
Furthermore, the activation sequence can strongly effect reactions. Many reaction-based systems operate with a modified IGOUGO - i.e. Side A moves his units one by one, then Side B reacts to each unit (cognizant on other factors such as limited number of reactions, etc). There's an argument that alternate move (Chess style unit by unit activation) allows a kind of organic reaction anyway. For example, in Infinity you can activate a unit multiple times in your turn, so having unlimited, powerful reactions balances the game, preventing powerful units from "ramboing" around and soloing the enemy army single-handed.
Also, unit reactions (i.e. squads in a platoon level game) can be different than individual reactions (i.e. single based minis in 1:1 skirmish).
Finally, are reactions best suited to particular eras? i.e. WW2-modern-hard sci fi rules seem to most commonly have reaction systems - where use of cover and firepower is paramount and melee combat is deemphasized. In this post I look at how a hard-core reaction system like Infinity may be integrally unsuited to medieval/fantasy because of the inherent gameplay style it encourages.
Also, are reactions better suited to particular "levels" of game? For example, I see reactions commonly used in platoon/skirmish games but less commonly at company/battalion/army level games (admittedly I don't play many of the latter).
Reactions may be the new hotness, but are reactions always good? Are they worth the potential speed/maneuver tradeoff? Does it bring in enough involvement/decision points to justify added complexity? When should we ditch them? What is the best way to limit them? Limit the range, the amount of possible reactions, or maybe the triggers?