Simplified Early Firearms

Simplified Early Firearms

Description: Match-, wheel-, or Flint-Lock weapons take too long to load to be reloaded in battle without a hireling to load it, thus they may be fired only once per battle unless the character also carries (potentially explosive) pre-prepared shot, wadding, and powder bundles (called "apostles.") 

Find it in: Lamentations of the Flame Princess

Keywords: Exploration, Fast-paced, Gritty, Pulpy, Rules-light, Swashbuckling 

Genres: Crime, Dark Fantasy, Pulp

Component Advancement

Component Advancement

Description: Instead of gaining entire levels at once, a character earns one component of their next level at once. For example, hit points, spell slots, re-roll pool increases, etc. are gained separately, creating a gradual power increase.

Find it in: Low Fantasy Gaming, Cypher System

Keywords: Rules-light

Genres: Any Genre

Roll the Body

Roll the Body

Description: When a character is knocked out of commission (such as at 0hp), their status is unknown unless an ally checks on them to see if they are living or dead. At that time the character makes a single saving throw or Luck Checks to see if they are alive, at which point they can be restored to consciousness.

Find it in: Dungeon Crawl Classics RPG, Low Fantasy Gaming, Troika!

Keywords: Cinematic, Dangerous, Gritty, Pulpy, Rules-light, Tense

Genres: Crime, Low Fantasy, Pulp, Western 

Lucky Escape

Lucky Escape

Description: Used in TTRPGs that use a Luck Scores. When a party of PCs is attempting to fled a battle they make Luck Checks, if more than half the party succeeds, then the party shakes their attackers and winds up in a relatively safe location that the GM chooses. Alternatively, everyone who succeeds escapes. This removes the need for chase mechanics.

Find it in: Low Fantasy Gaming

Keywords: Cinematic, Fast-paced, Pulpy, Rules-light, Swashbuckling

Genres: Crime, Low Fantasy, Pulp, Sword & Sorcery, Western

Advantage/Disadvantage as Dice

Advantage/Disadvantage as Dice

Description: Rather than rolling twice and taking the higher result, you roll an extra d6 and apply it as a bonus to your current roll. Likewise, disadvantage involves rolling 1d6 and subtracting it from the roll.

Find it in: Shadow of the Demon Lord, Rogue 2e

Keywords: Fast-paced, Rules-light

Genres: Any Genre

Special thanks to Scott Weber for pointing me to examples of TTRPGs that use this as a core mechanic. 

Pact Magic

Pact Magic

Description: Spells are replaced by relationships with spirits that have broad spheres of influence. The spirit may be called on to create any plausible effect that fits with its domain, allowing PCs to come up with inventive magical effects within a narrow scope. 
Find it in: Pacts and Blades, World of Dungeons, Aer
Keywords: Rules-light, Dark, Pulpy, Weird
Genres: Sword & Sorcery, Low Fantasy, Pulp, Horror

Abstract Coin

Abstract Coin

Description: Treasure is abstracted from numbers of coin, gem, jewels, and artifacts into a Wealth statistic that a character checks against to buy items, often depleting on a failed roll. Often used in conjunction with depletion dice. 
Find it in: the Black Hack, Tombpunk
Keywords: Rules-light, Epic, Pulpy
Genres: Sword & Sorcery, Epic Fantasy, Pulp, Superhero

Simplified Wounds

Simplified Wounds 

Description: Hit points are abstracted either into the number of Wounds that the target can take (in OSR games this is equal to HD). Any hit doing 4 or more damage deals a wound.
Find it in: Dungeoncraft
Keywords: Rules-light, Heroic, Fast-Paced
Genres: Any

Player-Facing Defense

Player-Facing Defense

Description: Game masters do not roll for attack, instead PCs have a defense ability they roll when attacked to beat a set attack value from a monster. 
Find it in: Knave, Maze Rats, Four Against Darkness, the Black Hack
Keywords: Rules-light, Heroic, Tactical, Player-Facing
Genres: Any

Zones - Subjective

Zones - Subjective

Description: Movement, reach, attack ranges, and spell effects are measured in zones: close, near, and far. Each is a relative distance from the character currently acting. So, close is close enough for the character in question to touch or engage in melee after just a few steps. Near is within range for a short-range spell a reach weapon or a sling. Far is something distant enough to require a bow or spell to attack and is too far to walk to and act in the same round. In some games this is interchangeable with objective zones. 

Find it in: the Black Hack, ICRPG Core 2e, Tiny Cthulhu
Keywords: Rules-Light, Fast-paced
Genres: Any

Zones - Objective

Zones - Objective

Description: Distance is abstracted into either three zones: near, far, and close or into two zones: here and there. The center of an encounter with the most action is counts as close, being at range to use short-range weapons and reach weapons places you at near, while ranged weapons and Magic attacks only places you at far. Or, being close enough to use a melee weapon is here, and being far enough away to use a ranged weapon is there. Movement, magic effects, Etc are abstracted to to work with this system. In some games this is interchangeable with subjective zones. 

Find it in: the Black Hack, Tiny Dungeon 2e, ICRPG Core 2e, 5e Hardcore Mode
Keywords: Rules-Light, Fast-paced
Genres: Any

Depletion Dice (Light)

Depletion Dice (Light) 

Description: Using a modification of the depletion dice, combustible light sources can be given a variable duration. 
Find it in: The Delve 2e
Keywords: Rules-Light, Fast-paced, Dark, Tense
Genres: Sword & Sorcery, Western, Low Fantasy, Post-apocalyptic, Horror 

Depletion Dice

Depletion Dice

Description: Instead of tracking individual items like money, rations, and ammunition, a die can be expressed as a die. Each time the resource is taxed, the die is rolled to test whether that resource has been reduced in quantity, which is represented bt replacing the die with a smaller one on a dice chain. After a d4 fails a roll to avoid depletion, the resource is lost. Offers a visual way to track resources. 

Find it in: the Black Hack, Tiny Dungeon 2e 
Keywords: Rules-Light, Fast-paced, Cinematic, Pulpy
Genres: Any

Effort Pools

Effort Pools

Description: Setting a hit point total for a challenge that requires time and multiple rolls can be expressed as a hit point total. Characters' actions can thus be expressed as damage toward the total, making a single consistent mechanic for all tasks. 
Find it in: ICRPG Core 2e, 
Keywords: Rules-Light, Fast-Paced
Genres: Any

Room Target Numbers

Room Target Numbers

Description: Set a single d20 target number for all tasks in a room or for an encounter, allowing for modifiers based on the difficulty of the task being rolled for ease of tracking and play. Often made visible to players. 
Find it in: ICRPG Core 2e, 
Keywords: Rules-Light, Fast-Paced
Genres: Any

Re-Roll Pools

Re-Roll Pools

Description: Player's have a refreshing pool of times they may re-roll checks, applicable only to specific abilities of the character (like proficiencies), or Luck Checks. Replaces skill system and saving throw specialization. 
Find it in: Low Fantasy Gaming
Keywords: Rules-light
Genres: any

Encumbrance Slots

Encumbrance Slots

Description: Instead of measuring gear by weight, give PCs a limited number of slots, either fixed or stat-based, in which items can be carried. Small items might fit in a single slot. Armor and very large items in multiples. 
Find it in: Lamentations of the Flame Princess, Knave, Maze Rats, Mausritter 
Keywords: Rules-light, Cinematic,
Genres: Any