I'm not an expert by any means but from my small investigations into D&D retroclones, I've learned there are a lot of differences between all the different editions of D&D. This geeklist is to help show those differences.
Since I am in no way an expert, please help make this geeklist better by commenting on each edition.
The original edition of Dungeons & Dragons (sometimes called 0e or OE or OD&D) had pretty simple rules. It started with 3 books Men & Magic, Monsters & Treasure, and The Underworld & Wilderness Adventures) and then there were supplements that expanded on the rules. There weren't a lot of classes (the first 3 books only had fighters, clerics, and magic-users) and spells were quite simple, usually only a short paragraph describing them. This edition also heavily referenced Chainmail, TSR's fantasy miniatures wargame.
A lot of the rules in original D&D were pretty vague so a basic set was needed. This is the Holmes Basic set, released in 1977 just before AD&D. This was mostly just a rewrite of OD&D to make the rules clearer but a few rule changes were made for simplicity's sake.
Shannon Appelcline wrote:
"When Eric Holmes put together the original Basic D&D, his purpose was simply to clean up and organize the original Dungeons & Dragons (1974) along with some content from Supplement I: Greyhawk (1975).
"He wanted to create a game that was easier to learn (as the original D&D was considered notoriously bad in that regard) and that could be better understand by the high school and junior high demographics, toward which the game was then trending. However, the expectation was that players would go on to the original D&D games from there. Basic D&D was never expected to be its own game system - at first."
The first book to come out for AD&D 1st edition was actually the Monster Manual. The rules came out the following year in 1978 and 1979 (for the Dungeon Masters Guide). AD&D added a ton of new races, classes, and many detailed rules to the game. Combat now had phases, weapons had different damages for different targets, and spells were much more complicated with required components.
In 1981, another basic set was released designed by Tom Moldvay. This ruleset pretty much ignored AD&D while continuing the work done by the Holmes set. The D&D rules were comprehensive, easy to understand and stayed streamlined, unlike AD&D. This set along with the Expert set designed by Stephen Cook are called B/X. Together, they brought a character from level 1 to 14.
This edition also introduced race as class. If you were a dwarf or an elf or a halfling, you didn't also get to choose a class; you just got the abilities of being a dwarf, elf, or halfling.
Also introduced in this edition: morale ratings for all the monsters.
Shannon Appelcline wrote:
"Whereas Holmes' Basic D&D was mostly a matter of organization and explanation, Moldvay's Basic D&D also engaged in simplification. Thus, for example, there were no longer separate character classes and races. The twelve race-and-class combinations of Holmes' Basic D&D (including things such as the elven fighter/magic-user multiclass) became just seven classes in the new Basic D&D: clerics, dwarves, elves, fighters (which had still been "fighting men" under Holmes), halflings, magic-users, and thieves.
"Moldvay's second edition also cleaned up character alignment, constrained spell choice, and even improved the layout of the book. All around, every effort was made to upgrade the game for starting players. As for the results, even former editor Holmes said, "I think the new Basic Set rules are an improvement over the first edition. Not a big quantum jump ahead, but better in a number of minor ways."
"Moldvay's Basic D&D was enough of a change from the previous edition of the game that it was actually a "new edition" as it's understood in the modern roleplaying market, which was a pretty rare occurrence in the 70s or early 80s.
"The Basic Set was (as planned) released simultaneously with the new Expert Set by David "Zeb" Cook, which expanded Basic D&D to levels 4-14. Gary Gygax mentioned a "Masters Set" around the same time, which was to cover levels 15-36, but that wouldn't appear during Basic D&D's second edition.
"Color-Coding the Boxes. Some people like to classify the D&D boxes by color: This edition is thus the "magenta box," to differentiate it from the "red box" edition that would follow in 1983."
In 1983, yet another Basic set was designed by Frank Mentzer. Along with the new Expert set, Companion set, Master set, and Immortals set, this brought characters up to level 36. This edition (called BECMI) also had race as class like B/X. This edition also introduced weapon mastery rules.
Shannon Appelcline wrote:
"By 1983, Basic D&D had gone through two major editions. The first was edited by J. Eric Holmes (1977) and was essentially an introductory set for the original D&D game (1975). The second was edited by Tom Moldvay (1981); it was the first truly standalone version of Basic D&D, and the start of the short-lived (but well-known) "B/X" edition.
"Frank Mentzer's version of Basic D&D, which would come to be called the BECMI edition (1983-86), was thus the third edition - or fourth, if you count original D&D as part of the sequence of games. BECMI would also be the most long-lived edition of Basic D&D, lasting almost eight years from the publication of the this Basic Set until it was superseded by The New Easy to Master Dungeons & Dragons Game (1991) and the Dungeons & Dragons Rules Cyclopedia (1991).
"A New Introductory Game. Mentzer claimed that the main reason behind this new edition of Basic D&D was that previous versions "were not 'revised', merely 'reorganized.'" He clearly wasn't talking about the mechanics, which demonstrably had been revised in Moldvay's version of Basic D&D, but instead how the game and its rules were structured. Mentzer's version of Basic D&D thus made some large changes to how the game was taught and presented.
"Menzter's first two goals for the new Basic D&D were to make the game approachable by beginners and to make it learnable from the rules. Mentzer's Basic Set is thus laid out almost as a tutorial, with new rules and concepts being introduced to players very carefully; the rules about GMing are then introduced only after all of the basic player concepts have been discussed.
"Mentzer also had three general goals for the new Basic D&D: it should be fun, playable, and true (i.e., to the spirit of D&D).
"A New Art Design. Mentzer's Basic D&D took advantage of the new "Product Finishing" Department at TSR, whose goal was to make TSR's books look as good as possible. You can best see their work through the upgrades to the trade dress of D&D that occurred in 1983. However, it's also very obvious in the Menzter Basic D&D, which is full of attractive graphic design (for the era), as well as artwork that's all by iconic D&D artists Larry Elmore and Jeff Easley.
"Color-Coding the Boxes. Some people like to classify the D&D boxes by color. This is thus the "Red Box," to differentiate it from the "Magenta Box" edition, which was the previous edition edited by Moldvay."
In 1989, AD&D 2e was released. This edition simplified some of the rules in AD&D, while expanding greatly on the non-weapon proficiencies (skills). Many supplements were released for this edition expanding the options for races and classes to a huge number. Half-orcs were no longer a player race and demons & devils were renamed to something else.
There were also three Players Option books released for 2nd edition that gave tons of optional rules to add: Combat & Tactics, Skills & Powers, and Spells & Magic. Some of these optional rules were incorporated into 3rd edition.
In 1991, the Rules Cyclopedia came out which compiled the Basic, Expert, Companion, and Master rules from BECMI and also added dropped the Immortals rules in favor of pointing players to the Wrath of the Immortals set.
Shannon Appelcline wrote:
"The Rules Cyclopedia is a compilation of the D&D Basic Rules Set (1983), the D&D Expert Rules Set (1983), the D&D Companion Rules (1984), and the D&D Master Rules (1985). It contains not only the rules from those boxed sets, but also the monsters, making the Cyclopedia one of two great sources for Basic D&D monsters, the other being the Creature Catalog (1986, 1993). Rules for skills and magic item creation from the "GAZ" Gazetteers (1987-91) are also included, making the Cyclopedia a truly massive compilation of about a decade's worth of Basic D&D rules.
"The Immortals Rules (1986) are notably not included in the Cyclopedia, although it does contain seven pages from the Master Rules that include basic information for immortals, including rules on PCs ascending to those lofty ranks.
"Rules on jousting in tournaments and on artifacts were also left out of the Cyclopedia."
In 2000, D&D 3e came out. This was also called the d20 system and revised in 2003 for the 3.5 edition. This was a dramatic change to prior editions as characters were made much tougher with more powers and could gain more powers (feats) as they leveled up.
The d20 system changed how things were rolled too. Before 3e, lower numbers were better numbers for Armor Class (AC). In 3e, higher numbers were better. Also, skill rolls had a Difficulty Class (DC) that you had to beat when rolling a d20. In general, higher was lways better and everything was a d20 roll (except for weapon damage and certain spell effects).
Saving throws also changed in this edition from 5 different saves in prior editions to only 3 in this edition: Fortitude based on Constitution, Reflex based on Dexterity, and Willpower based on Wisdom.
In 2014, 5e was released. 5e went back to 3e and simplified it so there were less feats. Also, the advantage/disadvantage system was used so that if you had advantage on a roll, two d20s were rolled and you kept the best but if you had disadvantage, you had to keep the worst. Many additions and subtractions to rolls were eliminated using the advantage/disadvantage system.
Saving throws were also changed in this edition so that each attribute has its own saving throw.