Anti-Hoarding: The game has encumbrance determined by a character's strength score. Obviously, the higher said strength score, the more they can carry. Certain factors such as size and form altered this number. Then, when the character exceeded this encumbrance, they start suffering penalties the more they carried over the limit. Eventually, if the total weight they're carrying got too high, they wouldn't even be able to lift it. Then again, D&D basically invented the Bag of Holding trope.
In the 1st Edition Dungeon Master's Guide section on siege weapons, catapults (light and heavy) and trebuchets all have minimum ranges that they can fire at.
In later editions, firing a bow provokes an attack of opportunity. Because giving your opponents free shots is bad, this means that minimum range is outside of your opponents reach.
In 5th Edition, a ranged attack has no minimum range. However, if it is performed within 5 ft of any enemy, then the attack is made with disadvantage, meaning that you roll to hit twice and take the lowest score, so the effective minimum range is 5 ft.
Basic D&D. In the Holmes (1977), Moldvay (1981) and Mentzer (1983) Basic sets and the Rules Cyclopedia (1991), magic users cannot wear armor.
In 1st and 2nd Edition Advanced D&D, magic users/wizards are simply forbidden to wear armor under the standard rules.note Unless multi-classed with an armor-wearing class - M-U/Thieves can wear leather, M-U/Fighters & M-U/Clerics can wear any armor (1st Edition only). There were exceptions made in later supplements, such as 2nd Edition class kits which allow a wizard with that kit to wear armor.
In 3.X Edition arcane casters can wear armor if they take a proficiency feat, but if they do they risk a percentage chance that the spell will fail to cast, justified as the armor interfering with the gestures involved in spellcasting. Bards and the add-on classes warmage and warlock can wear light armor without hitting this restriction, and can take a feat, "Armored Caster", to be able to wear medium armor without risking spell failure. Of course, a wizard with skill in the schools of transmutation and abjuration doesn't necessarily need armor since they can protect themselves quite well with their spells.
Druids are only allowed to wear armor (and other equipment) made from "natural" materials (wood, hides, stone, etc.) or else their powers are unusable. With just the core rulebooks (Player's Handbook, Dungeon Master's Guide, and Monster Manual), this restricts druids to wearing relatively weak armor (leather armor and wooden or leather shields in Original, BECMI, AD&D 1st Edition, and AD&D 2nd Edition; nonmetal light and medium armor and wooden bucklers, light shields, and heavy shields in 3.x Edition). Some of the 3.x Edition add-on books added some esoteric materials that are classified as natural and can be forged into heavier armors.
In 4th Edition, there's no such thing as arcane spell failure, but wizards still have the worst armor proficiency. They simply don't care about proficiency because (as of Player's Handbook III) they can take a feat to have AC equivalent to leather and still wear those wonderful magic robes made specifically for them.
5th Edition retains the "no arcane spell failure" ruling from 4th Edition and the mechanic has been simplified to "if you're not proficient in the armor you're wearing, you can't cast magic": meaning that while spellcasters can still wear armor that they aren't proficient in, the unfamiliarity of wearing said armor distracts them from being able to cast spells. Druids are still restricted to using "natural"/nonmetal armors and shields, but the Player's Handbook doesn't give a reason. The March 2016 Sage Advice column finally said that it's taboo, but that the player and DM can discuss how flexible said taboo is to the point of breaking it. That said, DMs can also house-rule a number of the esoteric materials from the 3.x Edition supplement books as well.
Artificial Gravity: The starship's technology in the 1st Edition module S3 Expedition to the Barrier Peaks.
Attack! Attack! Attack!: Most "striker" classes have a smattering of Utility abilities, or heavy reliance on stealth, that make them well-rounded. Essentials, or just someone well on their way to Munchkin land, usually focus on their basic attack, buffing it to high hell and/or getting out multiple attacks a turn, to the detriment of other abilities and feats. One good warlord and a party of 3-4 Essentials characters can get in the vicinity of fifteen to twenty attacks in a single round.
Balance, Power, Skill, Gimmick: The recommended party build in most editions is to have a warrior, arcane caster, divine caster, and skillmonkey (usually a fighter, wizard, cleric, and rogue, though there are any number of other permutations). Which character is which depends on how the individual characters are built, but the arcanist is almost always the Power and the skillmonkey is usually the Gimmick. If the party has a fifth character, this person can choose to emphasize any role and becomes the new Gimmick.
Banishing Ritual: Early editions had spells such as Banishment, Dismissal, Dispel Evil and Holy Word, all of which could send evil creatures back to their plane of origin.
In all editions, shields improve your Armor Class score, and the better one's AC score, the harder they were to hit.
Armor itself can fall under this trope, since depending on how the narrative describes a failed attack, it can be attributed to armor simply absorbing the potential damage without any loss of effectiveness. This is especially true in editions that have no sundering rules, whereby a worn suit of armor is effectively indestructible by mundane means.
If successful, the Block combat option will prevent a melee attack from hitting the character using it. note 2nd Edition Player Option: Combat & Tactics supplement
Cannot Cross Running Water: In 1st and 2nd Edition, vampires can cross running water, but if they're immersed in it for 3 minutes, they are destroyed. In 3E, they can no longer pass over running water on their own, but can be carried over it in a container. Also, they're not destroyed by immersion in running water if they have a swim speed before becoming a vampire. 5th Edition vampires are similar to 1st and 2nd Edition vampires; they can cross running water but take a flat 20 hit points of acid damage if they end their turn in it.
The 1st/2nd Edition compatible supplement The Tome of Mighty Magic has a spell called Divine Retribution. If the spell fails, it drains a level of experience from the caster.
The 1st Edition spell Energy Drain. There is a 5% chance the caster will be drained of a level of experience.
Energy Drain returns in later editions as a debuff, in the form of negative levels. At first, energy drain inflicts a temporary negative level, sapping hit points and inflicting a 1:1 penalty to all rolls and statistics. If this level isn't removed, and the save against it is failed the following day, the negative level may become permanent, forcing the hapless character to earn their level back the hard way.
Some rituals in 4e require spending healing surges, sometimes in addition to component costs. These surges can be contributed by those assisting in the ritual.
An epic destiny in 4e provides a higher-risk option: Elf High Mage, which allows empowering a spell using hitpoints if the wizard has no healing surges left.
Cast from Lifespan: In several editions, there are high-powered spells (e.g. Wish, Alter Reality, Haste) which age either the caster or recipient. In 1st edition, magical aging forces a system shock check, of which the penalty for failure is death(!), causing this to potentially cross over with Heroic R.R.O.D..
Cast From Stamina: In early editions of Advanced D&D, casting some spells costs points of Constitution. For example, casting the Identify spell lowered Constitution by eight points. The DM could require a character to make a Constitution check, and failing the check meant the character would become fatigued and have to rest.
Except for 4th edition, spellcasting is a liability in early levels and grows to engulf the entire game by the time you reach higher levels. Early on, your hit points are very low and even the fighter can go down to a single lucky critical hit from a tough opponent, making heavy armor and good hit dice a real boon. By the time you reach mid-game, however, your spellcasters will have obtained a stockpile of very useful spells that let them pull their weight, and by endgame a caster's buffing capabilities combined with their hundreds of spell slots filled with powerful, reality-altering spells have completely changed the game.
In 3.x Dungeons and Dragons, AC (Armor Class) is important and low-to-mid levels because it allows you to avoid taking damage from enemy attacks. At higher levels, either everybody has such high attack bonuses that AC becomes meaningless, or use attacks that largely ignore AC, like Saving Throws or Touch AC. Even if you have +5 Full Plate Armor most enemy attacks are going to hit you.
The changing gameplay priorities are built right in to 4th edition in the form of tiers. Every 10 levels, your characters get a pretty significant growth in power plus new capabilities, such as flight, teleportation or, in the final tier, the ability to cheat death and resurrect themselves at least once per day. There are some pretty dramatic differences between the capabilities and priorities of a heroic-tier party, where resources are scarce and powers need to be carefully rationed, and an epic-tier party, who won't flinch for anything short of a mad god and who can fight regular enemies for days straight without resting.
Character Alignment: invoked The Trope Namer. Its variant is so ubiquitous that the system from the second, third, and fifth editions of D&D is described in detail on the trope page. This is simplified in the 4th Edition from the nine-point axis to an alignment line of five alignments: Lawful Good, Good, Unaligned, Evil, and Chaotic Evil. "Neutral Good" and "Chaotic Good" are compressed into simply "Good," and likewise "Lawful Evil" and "Neutral Evil" are "Evil." Most ordinary people in a setting are presumed to be unaligned. The old alignment system has been spread through Memetic Mutation, however, so that it's not unusual to see nine-point alignment charts of, say Scrubs characters. Classic nine point alignment returned in 5th edition, and added Unaligned as a tenth to represent the alignments of beings not intelligent enough to understand morality.
Class Change Level Reset: In early editions, humans (and only humans) can "dual-class", losing most of the abilities of their first class and leveling up in the second. Once they reach the same level in the second class, they get the abilities of the first class back.
Continuing Is Painful: In early editions, resurrection magic is expensive, a permanent drain on your Constitution, and has a chance for failure that will result in Final Death. 3rd edition lightened up a bit by allowing a character to lose a level (which is easier to regain than lost Constitution), and 4th averts this mostly by scaling the cost for raise dead spells and inflicting only a temporary penalty to die rolls.
The spellstrike spellnote 2nd Edition, Forgotten Realms setting can negate an opponent's spell as it is being cast.
3rd Edition has a Counterspell mechanic. A prepared spell may be cast to nullify another caster's attempt to use the same spell. For example, a fireball can counter another fireball (but not delayed blast fireball, which is a different spell). Some spells are specifically opposed to and counter other spells (haste and slow may counter each other as well as themselves). Finally, dispel magic can be used as a universal counterspell but requires a unique "dispel check" to make the attempt. This worked very well with the base list of spells in 3.0 when, for instance, basically any caster intending to deal damage with a 3rd-level spell will have either fireball or lightning bolt slotted. As expansions piled up, however, the sheer length of the spell list made guessing a specific spell to counter essentially impossible, and the suggested house rule of countering with any spell of the same school (general thematic category) and same or higher level became so common that the Pathfinder mod/patch to the system uses it by default.
While technically not a spell, bards in 3rd Edition have the countersong ability, which lets them counter spells that depend on sound within a 30 foot radius. 5th Edition changed the name to countercharm and grants the bard and any allies within a 30 foot radius advantage on saving throws against becoming charmed or frightened.
The Thief (1st and 2nd Edition) and Assassin (1E) classes are this. The thief can do up to 5 times normal damage with a backstab, and the Assassin can kill an opponent in 1 hit by performing an assassination attack. Neither class is as good as a fighter in normal combat, due to armor restrictions and a lower chance to hit.
Third edition has weapons with an increased critical hit range (chance to make a critical hit), due either to their physical nature or magical enhancements. There are spells (like keen edge) and feats that do likewise (e.g. "Improved Critical"). A character can concentrate on gaining as large a critical hit range as possible (though most of the time, different critical range improvements do not stack).
In D&D 4th Edition, many players who play Avengers will choose weapons and feats to take advantage of the fact that Avengers roll twice for every attack and pick the highest roll in order to maximize the chance for a crit and maximize crit damage.
In D&D 5th Edition, the Champion subclass for the Fighter is twice as likely as everyone else to deal a critical hit with an attack, and three times as likely at later levels. The Rogue's Assassin subclass deals a critical hit every time they hit an enemy that is surprised, and (combined with Sneak Attack dice) as such have some of the most reliable burst damage in the game.
Individual instances are subjective. However, if the Game Master or majority of the players are convinced that you performed one during a D&D Encounters session, you are awarded a "Moment of Greatness" and receive extra renown points (which are used to provide rewards.) The official requirement to obtain this reward is to do "something inventive, daring, or just plain cool during a session of play", with die-roll luck having no bearing on the action.
In 5th Edition, there's a mechanic called "inspiration", which is intended as something the DM can give a PC who does something particularly awesome. However, the rules on it are so vague that most groups either ignore it or don't understand who it works.
Damage Discrimination: Some creatures are immune or resistant to certain types of attacks. This can lead to tactics where one guy uses an attack that covers an area his allies are in, but his allies are unharmed (at least in comparison with the enemy) due to said allies being immune (or at least resistant) to the attack. In the Blood War devils sometimes spam fire attacks without regard friendly fire because devils are immune to fire (unless perhaps if the fire attack is hellfire, which no one is completely immune to).
Damage-Increasing Debuff: There are various powers that allow you to cause your enemies to take extra damage from attacks.
1st Edition has the Potion Miscibility Table, which has a variety of results when someone drinks two magic potions. The bad results range from mild poison to lethal poison to an explosion inside the victim.
5th Edition includes a variant rule based on the 1st Edition Potion Miscibility Table. Worst case scenarios being the previously mentioned explosion and various poisonings, with the "best" case scenario being either one of the potions having their effect and duration doubled or one of the potions becoming a permanent effect. Although said permanent effect can be removed at the DM's discretion.
Determinator: In all editions, there are characters and monsters who can fight while at negative hit points, but it comes up more frequently with 3rd's feats and prestige classes. 4th edition gives most Epic Destinies (and thus most level 20+ characters) a means to cheat death daily, either with instant healing, a sudden transformation (like into a platinum dragon or a spell-slinging spirit), or a simple self-resurrection seconds later.
Diagonal Speed Boost: In 4th edition. To simplify the movement rules, moving one square diagonally counts as one square, and one square only, leading to the speed boost. Most of the earlier editions have a slight diagonal speed penalty, in that moving one square diagonally counts as 1.5 squares.
Dragons are known to interbreed so much that it looks as though many sorcerers had a dragon ancestor. The Half-Dragon Template can also be added to almost anything that can breed.
Members of the Sorcerer class are usually said to receive their powers from draconic blood, though a number of other supernatural ancestors are also possible. The Dragon Disciple Prestige Class allows a Sorcerer to tap deeper into their bloodline to gradually transform themselves into a Half-Dragon.
3rd Edition D&D eventually added a whole creature subtype for beings descended from dragons - the dragonblooded - with access to unique dragon-themed feats and Prestige Classes (including wings and Breath Weapons). While Sorcerers are not automatically dragonblooded, they can trade away some of their normal abilities in order to count as such.
Kobolds from 3rd edition onwards are depicted as tiny reptilian servants of dragons, with a tendency to become Sorcerers. Occasionally their draconic blood manifests particularly true in the form of a Dragonwrought Kobold, whose scales shine in the colour of their ancestor. While no inherently stronger than a normal Kobold, they are usually higher level and more likely to possess dragon-themed abilities. In addition they are biologically closer to Dragons than Humanoids, making them immune to certain spells and preventing their bodies from weakening as they age (the latter of which makes them extremely popular among spellcaster players, since old age provides bonuses to spellcasting-related stats that are normally Awesome, but Impractical).
Dump Stat: Basic D&D permits a limited means to reduce one stat to raise another, but only allows reducing strength, intelligence, and wisdom. Of those stats, strength increases melee damage, intelligence gives additional languages, and wisdom affects saving throws against spells. Stat dump is safe with early characters, but additional rules (e.g. ability checks, skills, etc.) change this.
Evil Makes You Monstrous: The optional 3rd edition book Heroes of Horror presents a system that implements this trope. In this, you don't even have to commit evil acts. Evil is practically a hazardous material, and turns you monstrous either physically (called corruption) or morally (called depravity), depending on the circumstances, if you don't take adequate protective measures or make your saving throws if you are around it.
In the 3rd edition, multiclassing incurred an XP penalty unless one of the character's classes was their race's "favored class". Humans got to choose their favored class.
1st Edition AD&D Dungeon Master's Guide, If a party of PCs has 10 times (or more) the hit dice/Character Levels of the monsters they're facing, the Dungeon Master should at least halve the Experience Points they receive for defeating the monsters.
Exploited Immunity: Any PC that is or acquires an immunity to a type of attack can be expected to exploit this (and a cunning DM will use it all the time):
Intelligent, spell-using undead (such as liches) can use spells that produce persistent effects over a large area (such as Cloudkill, Sleep, Stinking Cloud, etc) without worrying about being caught in the area of effect, as undead are immune to these effects.
Elven forces with mages can use Sleep spells with impunity, as they are mostly immune to them.
A mage under a Minor Globe of Invulnerability spell (which blocks 3rd level and lower spells) can use a staff which casts third level area effect spells at point blank range.
Flaming Meteor: Played with by the Meteor Swarm spell, which conjures small "meteors" that deal bludgeoning damage on impact and then explode for fire damage.
Game-Favored Gender: The earliest versions of the game give female characters a strength penalty. Not the case in later editions, which make gender differences purely aesthetic.
Geas: The 'geas' spell forces the player to fulfill a certain condition. There's also a divine equivalent named 'quest', which pretty much functions the same way.
Grappling with Grappling Rules: 1st Edition suffers from this the most, but only 4e has escaped the curse, and then only by completely removing all grappling in the core rules except for a single maneuver. However, there's an entire fighter subtype whose attacks revolve primarily around grappling.
High Voltage Death: 1st Edition module S3 Expedition to the Barrier Peaks. In Level II (Service Deck 5) there are areas that contain electrical equipment such as generators and transformers. If a PC decides to touch these devices with metal objects (e.g. weapons), there's a 10% chance that the character will be electrocuted and killed.
A character may attempt to control a Sphere of Annihilation with her will (Willpower check). Doing so successfully results in her temporarily controlling a tear in the reality that is capable of destroying pretty much anything in the universe on touch, No Saving Throw allowed.
From the same (and its derivative Pathfinder), Dominate Monster. Provided you succeed, you now have complete control over whatever's in front of you, which can be anything from a true dragon to a Hekatonkheires.
Hiroshima as a Unit of Measure: In 1st Edition, weights are listed not in pounds, but in gold pieces. 1 gold piece weighs 1/10 of a pound, thereby severely limiting the amount of money a party can haul out of a dungeon.
Hollywood Darkness: Generally averted, but played straight if your character happens to have low-light vision.
Home Field Advantage: Most creatures not native to the material plane exist on a plane that corresponds to their alignment. As such, the plane will give them an advantage over those that don't have the plane's alignment when they fight on said plane.
Hover Skates: 3.5 has a psychic power "Skate", which doesn't actually hover, but fits the trope otherwise by letting people skate along the ground instead of above it.
In D&D, improperly marking part of a summoning diagram allowed any creature summoned by that diagram to attack its summoner. An old piece of Dragon magazine art has a demon holding a magic-user in a Neck Lift over the diagram used to summon it. The demon is pointing to the diagram and saying "Missed a spot."
The Dragon magazine #147 article "Variety, the Spice of Magic" explains what happens when a spellcaster uses an incorrect material component to cast a spell. Effects include halving the range, area of effect or duration of the spell or increasing the target's saving throw (chance to reduce or negate the spell).
"Instant Death" Radius: A big problem in the 3rd Edition games is certain monsters being absolutely painful to approach via long melee reach and the Attacks of Opportunity provoked from trying to get close enough to melee them, which will usually hit for heinous amounts of damage due to their high Strength, such as any monster that's larger than you. The five-foot step rule of 3E, known as "shifting" in 4E, exists because of this.
Invisibility with Drawbacks: Most forms of invisibility available to PCs have some drawbacks to prevent abuse. The basic invisibility spell ends right after you make an attack or target someone with a spell, and items you pick up remain visible unless you can fit them in a pocket or bag. Improved invisibility is less volatile, but leaves a telltale that lets enemies know what square you're standing in.
Invisible Means Undodgeable: Attack spells such as Charm Person, Sleep and Power Word Stun are invisible and cannot be dodged or deflected with weapons.
Kevlard: There is a feat aptly called 'Obese' in the Book of Vile Darkness supplement. It increases constitution by 2 at the expense of dexterity, thus increasing your endurance.
Knockback: In the Player's Options version. It exists as a feat by the same name in 3.5, and a couple of others intended for large monsters in 3.0.
Laughing Mad: The Tasha's Hideous Laughter spell inflicts a temporary form of this.
Linear Warriors, Quadratic Wizards: First to third editions, averted in fourth, some would say it's back in fifth. It was, at least in gaming, the trope creator, and many games based on or inspired by D&D suffer from the same issue. It was at its worst in 3rd/3.5 editionnote although in terms of sheer power level, 1st Edition magic users were probably the most overblown. Prior to 2nd Edition, a fireball did 1d6 damage per level of the caster, with no upper limit. due to significantly lowered character mortality and rules specifically for starting higher level campaigns, leading to far more characters achieving high levels and thus encountering the issue.
Loads and Loads of Rules: While this applies to pretty much every published RPG ever, the rules for early editions of D&D are rather lengthy. Worse still, most of these rules are poorly organized.
To give some perspective, the rules for grappling run a whole two pages in the 3rd edition Rules Compendium. The rules for magic items weigh in at 5, and the rules for movement are covered by ten whole pages. And then you have Polymorphing rules, which have been changed so frequently that you need to check the errata instead of the most recently printed book just to make sure you are up to date.
4th Edition has tons of rulebooks. Originally, there was the Player's Handbook, Monster Manual and Dungeon Master's Guide. Then came the PHB2, PHB3, MM2, MM3, DMG2, Adventurer's Vault 1 and 2, Arcane Power 1 and 2, Martial Power 1 and 2, Divine Power, Psionic Power, Draconomicon Chromatic Dragon, Draconomicon Metallic Dragons, Demonomicon, a book full of undead creatures, as well as the obligatory Forgotten Realms, Eberron and Dark Sun Campaign Setting books. And chances are this isn't even a complete list. By the time 5th Edition hit the shelves, making a proper character for 4.0 practically always required bringing at least 5 different books to the table.
In the Forgotten Realms setting during 2nd Edition, certain clerics of Tymora, the goddess of luck, have the granted power to re-roll a die once per day. Similarly, some clerics of Beshaba, goddess of misfortune, have the ability to force enemies to re-roll their dice.
3.5 has the Fate Spinner Prestige Class, where you can shift around good and bad luck, as well as the Fortune's Friend, where having supernatural good luck and unlikely events is a class feature.
The Elf race in 4th edition has an innate power that allows the player to re-roll a single attack roll during an encounter, though they must accept the second result.
Most leader-type classes in Dungeons & Dragons 4th edition have powers that allows one to do this, such as the Bard's Unluck which allows him to swap an enemy good roll for a bad one and a friendly bad roll for a good one. Halflings have the power to force an enemy to re-roll a hit.
In 4th Edition Eberron, the Dragonmark of Detection allows one to roll twice on perception checks and pick the best result.
Dragon magazine #118 had an article on "Hero Points", which can be used to improve the chance of succeeding on a specific roll.
Eberron and the d20 system has "Action Points", which can be spent to increase the chance of succeeding on a roll.
In 5th edition, halflings can reroll any 1 on a d20 roll, while fighters and paladins have access to a weapon ability that lets them reroll any 1 or 2 when calculating damage.
Magic or Psychic?: Dungeons and Dragons has one of the more notable distinctions. While magic has a variety of sources, psionics always stem from the user's mind. And psi lacks verbal, somatic, or material components like spells usually do. In addition, in 3rd edition most magic-using classes function by Vancian Magic while Psionics are limited by Mana.
Some attacks deal "ability damage" that reduces the target's stats but heals when the character rests and "ability drain" which can only be healed magically. Constitution drain is essentially this trope, because a character's maximum hitpoints are calculated from it. A dead first level character who's resurrected (most resurrections cost the resurrected character at least one Character Level to avert Death Is a Slap on the Wrist) also permanently loses a point of constitution.
The Vargouille monster can do this. If the victim of its attack fails a saving throw vs. poison, the Hit Points of damage inflicted are lost permanently and can only be recovered by using a Wish spell. No form of healing magic will bring them back.
Epic Level Handbook: The Lavawight and Shape of Fire have the blazefire ability which does exactly that.
The Book Of Exalted Deeds has the Vassal of Bahamut Prestige Class, which uses bonus dice to deal permanent hit point damage to evil dragons.
In 5th Edition, the Witch Hunter/Blood Hunter class has an ability that lets them reduce their maximum HP by their level in the Blood Hunter class, and in return increases their damage by a certain amount. They regain their HP when they rest, however.
The psion, in any edition, is basically a wizard but with the gimmick of spell points instead of memorization. The sorcerer, in third edition, is a wizard but with spontaneous casting instead of memorization.
Similarly, warlocks, especially in third edition, exist pretty much solely to have a completely different magic system from everyone else, based around spells that they could cast infinitely and generally completely abandoning Vancian magic. They're so blatantly just there to be mechanically different that they don't even really get their own origin, just a slight word-swap of the sorcerer class description substituting fairies for dragons.
In Fourth Edition, where most of the earlier classes represent a particular fantasy archetype, most of the later classes mainly exist because of a certain mechanical gimmick, and share archetypes with an earlier class. For instance, the avenger (gimmick: roll twice for each attack, same archetype as paladin), psion (as above), runepriest (gimmick: switch between offensive and defensive mode at will, same archetype as cleric), and the fact that there are two different classes named 'assassin' both with a different gimmick.
In some early editions, the thief class itself, as the only one with explicit (percentile-based) skills, fits the bill.
Morale Mechanic: Early editions have a Morale score for each monster or NPC enemy, as well as Resist Fear saving throws. Failing the latter causes the monster to panic and run away. There are, however, fearless monsters, such as the basic undead that lack self-preservation instinct. They also have spells like Fear, which cause the same effects as regular panic attacks and can be resisted in the same way (albeit at a penalty).
Multiple Persuasion Modes: The Diplomacy/Persuasion and Intimidation skills. Although they have to be role-played very differently, they are quite similar mechanically: both skills are Charisma-based and their main difference is that the former can improve the NPCs' attitude to the speaker on a successful check, while the latter always worsens it. There is also the Bluff/Deception skill, which lets characters tell convincing lies, but unlike the other two social skills, its checks are always opposed by the target's Sense Motive/Insight skill.
Mundangerous: In 3rd edition, being on any surface (marbles most prominently) that requires something to balance without 5 ranks in the "balance" skill (which is otherwise not gotten as it's a rare class skill and most times you need to balance you can just fly), will result in being "flatfooted", a fairly big disadvantage, and it effects any land based foe without the balance ranks.
Non-Combat EXP: The game has various rules for GMs to give out EXP for completing tasks outside combat, such as talking one's way out of a fight or for superb roleplaying. Also, long before there were official rules for it, this was a very popular house rule. The earliest editions even gave out EXP for collecting loot, although it was limited to gold pieces and the conversion was "1 gp = 1 xp".
Noiseless Walker: Thieves could do this using the Move Silently skill. Boots of Elvenkind also allow the player to walk silently.
One-Handed Zweihänder: There's a feat in 3rd ed. Dungeons & Dragons called Monkey Grip that allows a character to use two handed weapons as one handed weapons. As the system is one of the bigger cases of Shields Are Useless, and there is an inherit damage boost to two handed weapon use and even with the feat you suffer a penalty, it's common to see comments on how bad it is.
Any spell in that lists its target as "object" will not affect creatures. A few do make an exception for constructs.
Leprechauns from 1st Edition Advanced Dungeons & Dragons had "polymorph non-living" as one of their most annoying talents.
Only Mostly Dead: In 1st edition, most poisons kill the victim on a failed saving throw. However, the 2nd level cleric spell Slow Poison temporarily halts a poison's action, "even causing a supposedly dead individual to have life restored" (from the spell description).
"Open!" Says Me: In 1st and 2nd Edition AD&D, even opening an unlocked door requires a character to roll against his Strength. The chance for an average-strength person to open an unlocked door is 1 in 3. You might have to bang yourself against the door several times before it'll open. Forcing opening a locked door can only be accomplished by someone super-strong (18/91 strength or higher), and even then the chance of success is quite small — and if you fail you can never try to force open that same door again.
Out of Continues: AD&D has limit to the number of times a player can be raised, in addition to a chance that the raise attempt fails.
Padded Sumo Gameplay: 4E combat is often called "Padded Sumo" by its detractors, as damage outstrips health, and many powers focus on moving enemies around.
In 1st Edition Advanced D&D a gold piece is worth 200 copper pieces. Many monster treasures have thousands of almost worthless copper pieces. Since moneychangers often charge a significant fee (e.g. 10%) for changing copper pieces into higher denomination coins, a PC might decide to pay for a purchase with bags full of coppers.
A module for Edition 3.5 has an example where doing this is to your advantage. You run across some barbarian halflings who use a barter system—which means 1 gold piece (weighing about 1 third of an ounce, or 7.5 grams) is not much use to to them. However, the equivalent in copper pieces (100cp = 1 gp in this edition) means 2 pounds of metal they can melt down and use.
Spell scrolls can usually only be activated by characters who can also access the spell through their class spell list (e.g. only a wizard or sorcerer can use a scroll of magic missile, only a cleric or druid can use one of flame strike, et cetera). This restriction can be overcome with a Use Magic Device check. Use Magic Device also lets you overcome several other restrictions, most of which fall under Level-Locked Loot.
Downplayed example: The Holy Avenger longsword is ordinarily "just" a +2 cold iron longsword. In the hands of a paladin it becomes a +5 holy cold iron longsword that also provides spell resistance to the wielder and anyone adjacent to them, as well as allowing them to cast greater dispel magic once per turn.
Proactive Boss: Count Strahd von Zarovich from the classic I6 Ravenloft module was one of the first (if not the first) such bosses in D&D, showing up in different corners of his eponymous castle to manipulate the heroes (if not to try killing them outright). The module was highly praised at the time for its intelligent and proactive villain, as most D&D modules until then (1983) had been little more than straightforward Dungeon Crawls where all encounters were either nailed to their rooms or completely random.
Psychometry: In D&D 3.5, Seers have access to a power called object reading that lets them touch an object and learn information about its previous owners. There's also a magic item called "gloves of object reading" that grants a similar ability.
Random Encounters: The Wandering Monsters tables from this game laid the groundwork for this trope.
Randomized Damage Attack: Almost all attacks that do physical damage (melee combat, spells, psionics, etc.) inflict a variable number of Hit Points. In fact it would be quicker and easier to list the attacks that always do the same damage. This is true of most games inspired by Dungeons & Dragons as well.
Ranged Emergency Weapon: In pre-4E D&D, almost every melee character needs to carry one of these. (In 4th edition, you often have powers that let you make ranged attacks with your "melee" weapon anyway.)
Regenerating Mana: In 1st Edition psionics work this way. Using psionic powers uses up the character's psionic strength points. Over time, the strength points are gradually recovered. The speed of recovery is based on how much the psionic exerted himself, from zero points/hour for hard exertion to 24 points/hour while sleeping.
Shed Armor, Gain Speed: Heavy armor increases AC, but does so at the cost of slowing movement and reducing the maximum dexterity bonus to AC. Ironically, a character who somehow gains a high enough dexterity score can end up having a higher armor class by not wearing armor.
Siege Engines: Rules were included for the use of siege engines from the start - see Chainmail above. Eventually the game invented a few new ones.
Songs in the Key of Lock: The 3rd Edition DMG mentions a note played on a lute as a possible key to open a magical door.
Sourcebook: Popularized the concept. Recurring titles include Manual of the Planes, Deities & Demigods, Draconomicon, etc.
Spell Crafting: While the games, post 2nd edition at least, tend to have very thorough options in the core rules, the Dungeon Master's Guide carries extensive information on making new magic and items.
Square Race, Round Class: Earlier editions put restrictions on race and class combinations, whether by disallowing, putting a level limit on them, or requiring minimum attributes. This was removed in third edition, with those combinations being hard but possible. Fourth edition removes all barriers, at worst giving no +2 bonus to the primary attribute. Fifth edition finally brings back attribute penalties, but only for kobolds (-2 strength) and orcs (-2 intelligence) as playable monster races from Volo's Guide to Monsters.
Straight for the Commander: In the 1st Edition supplement Unearthed Arcana. In battle, cavaliers will automatically charge toward and attack enemy leaders in an attempt to gain glory by defeating them. The charge will be made at full speed, regardless of army cohesion, intervening friendly troops, or any other consideration.
Summon to Hand: There are many powers which can achieve this effect, and many weapons with it as an implicit power.
Super Gullible: Characters with low Wisdom and Sense Motive/Insight modifiers tend to have trouble discerning truth from lies, and therefore often fall under this trope.
Supernatural Fear Inducer: Starting with 1st edition, quite a few AD&D monsters have the ability to cause fear as an innate power:
Various spells such as Scare and Cause Fear (the reversed form of Remove Fear) can temporarily frighten an opponent.
The 1st Edition psionic power Telempathic Projection allows the user to implant fear in an opponent's mind.
Certain creatures have the ability to cause fear in opponents, such as androsphinxes (roar), beholders (one of their eyes), some demons, devils, dragons (roar), mummies and satyr (by playing their pipes).
All dragons in some editions are constantly surrounded by an aura of fear that sends any low-level character into panic. They can, however, turn it off for a brief time. A few other monsters also have fear auras, such as liches.
In 3.5 and Pathfinder, it's not a magical or psionic effect: dragons are just plain scary. Mechanically, if a dragon with greater hit dice than you does anything threatening, you have to roll your save.
One of the beholder's ten small eyes can cause fear in the victim it looks at.
The demon princes Demogorgon, Juiblex, Orcus and Yeenoghu can instill fear in a target as one of their innate abilities.
The demon prince Graz'zt causes fear in all within 60 feet who see and hear his displeasure.
The Type II, Type III, Type IV and Type VI demons can act on other creatures as if using a Wand of Fear on them.
Babau, Bar-Igura, Baron and Marquis cambions, chasme, and rutterkin can cause fear by touch.
All devils have the ability to engender fear in living things. The exact details are different for each devil.
Arch devils inflict fear by looking at others. Asmodeus with his gaze, Baalzebul with his glance, Dispater by staring, Geryon by glaring, Amon, Mammon and Mephisto by gaze and Belial by stare.
Other named devils: Bael in a 20 foot radius, bearded devils and Titivilus by touch, Glasya and Hutijin by speaking to victims, and Moloch with a Breath Weapon.
Barbed devils cause fear by striking an opponent, bone devils generate fear in a five foot radius, erinyes affect anyone who looks at them, ice devils radiate fear in a ten foot radius, malebranche exude fear in a five foot radius, and pit fiends shed fear in a 20 foot radius.
All dragons of adult age (51 years) and older radiate fear by flying overhead or charging. Depending on how many hit dice the victim has, it can cause them to flee in panic, be paralyzed with fear or just take a penalty to hit.
Brass dragons have a breath weapon that consists of a gas that causes fear in anyone who breathes it.
When a dragonne roars, the sound can inflict fear on anyone who hears it, which will sap 50% of their strength.
Just seeing a lich causes any creature with less than 5 Character Levels or hit dice to flee in panic.
The sight of a mummy can make any creature paralyzed with terror.
A satyr can play music with its pipes to inflict fear on any opponent that hears it.
The roar of an androsphinx can create fright in any creature within 120 yards, making them flee in panic for 30 minutes.
Any being that looks into a yeti's eyes is rigid with fright for the next three combat rounds.
The Phantasmal Killer spell forces the target to save, or immediately die of fright.
Tap on the Head: 1E has the Monk class and the sap, and some d20 games also have a blackjack/sap.
Teleport Interdiction: Older editions have spells that prevent teleportation into an area, such as 'Forbiddance', 'Teleport Block' and 'Wall with No Doors'. 'Teleport Ward' (fiendish spell from Dragon Magazine) allows more protection with high magic resistance. 'Translocation Shift' (Dragon Magazine) redirects incoming teleporters to a different location. 'Dimensional Anchor' (PO Spells & Magic) blocks the affected being from being moved by any forms of teleporting and planeshifting. Anticipate Teleportation (D&D 3.5 Complete Arcane) while not blocking teleportation entirely, delays teleporters' arrival to allow ambushing them.
Every edition has the blink dog, a monster who teleport spams as a free action.
In 3.5, the totemist's blink shirt soulmeld gives teleportation every round of every day. Lots of teleportation spells and abilities can be combined with the Telflammar Shadowlord, who gets a full attack every time he teleports. All of this is brought to its apotheosis with the Chrono-Legionnaire build.
4E has the swordmage, whose Aegis of Assault teleports him to a monster who tried to attack his allies; the eladrin knight, who can teleport every time he hits something; the warlock, who has the at-will Ethereal Sidestep and the paragon path Evermeet Warlock to make himself invisible to anyone he teleports away from and bring his allies with him; and the bard, who can specialize in teleporting his allies and enemies. Warlock and bard are often combined to form what is known as the Bard Taxi.
Theme Deck: 4th edition gives a wide variety of powers for characters, but they can only select one for a given slot, meaning characters have to plan in order to advance. As a meta example, these powers are also printed on cards and put into a deck.
Read magic is one of the most basic spells available to any caster, and can be used to identify the spells contained in scrolls. The catch is that only an arcane spellcaster (bard, sorcerer, wizard) can use arcane scrolls; same goes for divine casters (cleric, druid, paladin, ranger) and divine scrolls.
Detect magic is another common, low-level spell, and combined with the Spellcraft skill a player can analyze the aura of a magic item to infer some of its properties.
Identify is a 1st-level spell usable by wizards, sorcerers, bards, and clerics with the Magic domain. It identifies all properties of a single magic item. You can scribe a scroll of the spell with the proper item creation feat. Another option is to use a Knowledge skill check to deduce the item's properties.
For more mundane treasures like gemstones and art objects, the Appraise skill lets a player estimate monetary values.
The rulebooks suggest that a character who frequently uses potions can learn to identify them by sampling the contents; just enough to taste but not enough to activate the magic.
In 4th Edition, item identification is safely performed during a short rest, with only rare or obscure items requiring an arcana check.
5th Edition includes three methods: using the identify spell, the "short rest examination" from 4th Edition, and just outright experimentation with said magic item. There is also a variant rule to make magic item identification more difficult by forbidding the "short rest examination" and requiring the use of either the identify spell, experimentation, or both to find out a magic item's properties.
Vancian Magic: D&D is the Trope Codifier for magic rules made by Jack Vance. Typically each spell is distinct, a spell can only do one thing without variation, and there's a limited number of spells that a magic-user can cast in a day.
Later editions have "magic" classes that are the exception, usually with their own unique rules, but with less books published than those for the original classes. Exceptions include Psionic classes, Incarnum classes, Warlocks, Binders, Truenamers, and more.
Vow of Celibacy: The 3E supplement Book of Exalted Deeds includes a "Vow of Chastity" feat, among several other "Vow of X" feats following the same basic format. This one in particular requires the character to abstain from both marriage and sex, and in return grants +4 to saves against charm and phantasm effects. Breaking it voluntarily costs you the feat's benefits permanently, while breaking it due to Mind Control costs you the benefits until you can get an atonement spell cast for you. (There's no ruling in the book on how it interacts with rape.) This feat is a prerequisite for the Beloved of Valarian Prestige Class.
Wishing for More Wishes: Although the "Wish" spell and items and creatures that duplicate its effect are notoriously limited, there is no rule specifically against wishing for more wishes. However, since the Dungeon Master is directly instructed to interpret gamebreaking wishes as a Jerkass Genie, attempts to do so tend to go badly. Examples include being locked in a Stable Time Loop experiencing the moment in which the character wished for more wishes an infinite number of times, being transformed into a Pit Fiend (which can grant one wish per year and has an infinite lifespan, but also happens to be Lawful Evil and likely to kill the rest of the party on general principle), etc.
Writers Cannot Do Math: With Basic D&D, normal movement speed is 120' per turn, which converts to about 2.5 inches per second. Although that edition justified that speed as a means to also ensure properly examining rooms, the same speed is applied whether exploring or instead backtracking to a previous location. The other movement speeds (e.g. encounter speed of 40' per round or 10 seconds) become an order of magnitude faster.
Your Mind Makes It Real: For some spells, usually illusion spells with the shadow sub school—although illusion spells with the shadow subschool still hurt you if you don't believe in them, just not as much, under normal conditions. Certain builds in 3.5, however, can make shadow duplicates of spells that are 160% real.