Early in the history of Dungeons & Dragons, the game’s creators made classes that rewarded strict rules of behavior with extra power. For instance, a player taking the challenge of playing a chivalrous cavalier always fought fairly, but also gained a bonus to attack with sword and lance.
Arguably, cleric stands as the first class limited by rules of behavior. Some holy men in history attempted to straddle the gulf between peaceful servant of god and spilling the blood of enemies by opting to bash their foes’ skulls while claiming not to spill blood. Based on that lore, original D&D required clerics to choose not to wield edged weapons or arrows. Back then, all weapons dealt 1d6 damage, so this limit only counted as a disadvantage because the treasure tables made magic swords 20 times more common than non-edged weapons.
Former Wizards of the Coast head Peter Adkison shepherded D&D’s third edition design. He wanted to purge “restrictions that did a good job of reinforcing play balance but still didn’t make sense.” Why would a cleric devoted to Apollo the Archer refuse a bow? For third edition, D&D’s designers replaced the rule that prevented clerics from using swords with a proficiency system that made swords a poor option. Modern D&D follows the same pattern, but it still includes a class that requires characters who choose to limit themselves. More on that at the end.
With D&D’s first Greyhawk supplement, Dungeons & Dragons co-creator Gary Gygax started adding classes that clearly balanced extra power with limitations that required certain behavior. Paladins started as a version of fighter who always acted lawfully. (D&D lacked good and evil alignments then.) “Any chaotic act will immediately revoke the status of paladin, and it can never be regained.” The paladin’s generosity required the class to give away inessential magic items and other loot. “Gifts must be to the poor or to charitable or religious institutions, i.e. not to some other character played in the game.”
The first editor of The Dragon, Tim Kask, wrote, “It was so rigid a PC class; that smacks of Gary’s fiendish wit. Make a character truly extraordinary and make it really tough to play in terms of always having to do the ‘right’ thing, ALWAYS.”
By the Unearthed Arcana book for Advanced Dungeons & Dragons, many classes offset power with a roleplaying catch. “Barbarians in general detest magic and those who use it. They will often seek to destroy magic items.”
Gygax knew this created a recipe for party conflict and embraced it. “When I played a barbarian, I would indeed attempt to get at newly discovered magic items and rid the world of their bane, and if some mage was foolish enough to flaunt such an object before the character, and he could lay hands on it, then…” When a monster turned Gygax’s barbarian to stone, it ended the character’s career. “No cleric or mage in the group was much interested in having the poor chap returned to life.”
In most games, players saw such restrictions as problems to solve with ingenuity. Part of the paladin’s reputation for lawful stupid comes from all the do-gooders conned into leaving the room when the thief wanted to question or kill prisoners. And how many barbarian players opted for characters just stupid enough to fail to realize that their +2 sword and cloak of protection happened to be magical? Sample dialog: “Tan-kor, you should wear this cloak we found because it, um, matches your eyes.”
Second edition introduced class kits that made characters more distinctive and powerful, often in exchange for behavior restrictions. For instance, the bladesinger from the Complete Book of Elves ranked as one of the most powerful kits. The catch: Having to protect elves whenever the opportunity comes. Players got a boost for simply committing to bite on a certain sort of adventure hook. A more onerous code weighed cavaliers, who always had to fight fairly and chivalrously. So if a cavalier’s foe slips off a cliff and hangs by the edge, the cavalier must help the foe up before resuming the fight.
Such restrictions only worked in campaigns where every player valued roleplaying. In a mix of actors and power gamers, behavior limits cause friction—even when the code doesn’t require destroying magic items. Players who value immersing into character grow annoyed by the players who just look for loopholes in codes of conduct. Dungeon masters dislike the role of mandating or penalizing behavior that ignores the hindrances built into a character kit.
By third edition, the D&D team mostly dropped roleplaying limits from the class-design toolbox.
Still, one code of conduct remains in the modern druid class. The fifth edition Sage Advice Compendium explains, “Druids don’t lack the ability to wear metal armor. They choose not to wear it. This choice is part of their identity as a mystical order. Think of it in these terms: a vegetarian can eat meat, but chooses not to.” This restriction brings enough teeth for class balance while proving clear enough to defy most players seeking loopholes.
Related: 4 Ways D&D’s Creators Tried and Failed to Balance Classes
Even though the megadungeons under Greyhawk and Blackmoor became the foundation of Dungeons & Dragons, such dungeons rarely see play anymore. Why not?
So rather than attempting to capture Greyhawk Castle, Gary opted to publish adventures that he had created for D&D tournaments at conventions. For instance, the official D&D tournament at Origins ’78 ran the 
The Ruins of Undermountain
3. Play styles expanded. Sometime in the middle of the 70s, for the first time ever, a party of adventurers visiting the inn met a hooded stranger with a job that needed doing. D&D expanded beyond a series of dungeon expeditions aimed at claiming treasure. Players began to favor games that mixed action with story. 
“This pattern of female equality also held true for the D&D play and roleplaying that took place in SF fandom—and that’s where I did my roleplaying. Not at hobby stores but at the LASFS and at science fiction conventions, usually with old friends or with people I’d met through A&E. A&E started through people who already knew one another through APA-L or through science fiction fannish connections.”
Starting with the original rule books plus the Blackmoor and Greyhawk supplements, Holmes made D&D comprehensible while keeping “the flavor and excitement of the original rules.” As much as he could, he reused wording from the original game. But J. Eric Holmes had learned to play D&D from the Caltech Warlock rules and he probably had seen The Perrin Conventions. That experience led him to pitch Warlock’s spell-point system to Gygax. We know how that turned out. Gary hated spell points. However, Holmes’s take on D&D included one West coast innovation: The character with the highest Dexterity struck first. Back then, monster stats lacked a number for Dexterity, so the rules explain, “If the Dungeon Master does not know the dexterity of an attacking monster, he rolls it on the spot.”
Using “feat,” a word for a stunt, as a game term for a character feature or talent bothers my wish for precise terminology. Back in the third-edition days, this word choice annoyed me to such an embarrassing degree that I griped about the misnomer on the Wizards of the Coast D&D boards. That post probably only exists on a backup tape labeled “GLEEMAX” in magic marker.
Compared to chalk, Magic Mouth offers more portentous way to deliver a message. Glyph of Warding adds a common magical trap. The spells weave useful magical effects into both the lore and the rules of the game. They give DMs ready-made tricks for their dungeons. Players enjoy recognizing these familiar bits of spellcraft mixed with the fantastic.