It's funny that most RPGs when releasing a new edition, the changes are mostly what D&D would call a .5 edition. It's mostly D&D (and d20 derived systems like Pathfinder) where editions are radical reinventions. I think if the changes between AD&D -> 3e -> 4e -> 5e weren't as large and invalidating, I don't think people would be treating edition changes as radioactive and WotC wouldn't be afraid to call it a new edition.
I actually think that, if I'm Hasbro, this is actually where I want D&D to live. We put out mild alterations, and minimal changes, and rules options, and nobody cares because it all works together. Like a new Monopoly set, if you buy the newest version, you get the latest and greatest, but it's still recognizable as
Dungeons & Dragons. And I think 5th-Edition was them realizing this fact. Let's look at the
actual editions that have come before...
1. Dungeons & Dragons (1974, aka OD&D, or "The White Box"): The game as it was presented in 1974. 6 ability scores, AC, hit points, 3 classes (Cleric, Fighting Man, & Magic-User), 4 playable races (Dwarf, Elf, Hobbit & Human). Later printings would change "Hobbit" to "Halfling" and supplements would add Thief, Paladin, Ranger and other classes and races before we got...
2. Dungeons & Dragons Basic Set (1977, aka "Holmes"): A re-presentation combining the rules in OD&D and
Supplement 1: Greyhawk, for the races of Cleric, Fighter, Magic-User and Thief, and the races of Dwarf, Elf, Halfling, and Human. It only covered the first 3 levels, after which it was assumed that players would move to...
3.
Advanced Dungeons & Dragons (1977, aka "AD&D" "1st-Edition" or "1e"): Expanded rules covering more classes and races than Basic (10 + bards, the game's first "Prestige class" & 7 (adding gnomes, half-elves, & half-orcs). AD&D established the paradigm for hardcover book releases (that would dominate most later editions) of a
Player's Handbook,
Dungeon Masters Guide, and
Monster Manual. AD&D was not fully compatible with "Holmes" and changed a variety of rules, increased the hit points die for (most) classes, and altered the ability score bonus tables and many other systems. Because of this, while supplements over the next several years continued to expand AD&D options, plans for the game to act as the only continuation from Holmes were scrapped, leading to the creation of...
4.
Dungeons & Dragons Basic Set &
Dungeons & Dragons Expert Set (1981, aka "Moldvay/Cook" or "B/X"): A total revision of the rules, preserving the simpler ability bonuses and lower hit points of Mentzer, but scrapping the concept of race and instead introducing the Dwarf (basically a fighter with some racial bonuses), Elf (a d6 fighter/magic-user hybrid) and Halfling (a d6 fighter with saving throw bonuses and stealth) classes. The
Expert Set would expand the available levels to 14th, with spells as high as 5th for Clerics and 6th for Magic-Users. Levels over 14th wouldn't be supported by B/X until after the release of...
5.
Dungeons & Dragons Basic Set (1983, aka "Mentzer" or "The Red Box"): A cleaned up version of Moldvay, the Mentzer Red Box didn't really change anything about the game other than cleaning up the presentation. However, the much better presentation of Basic and Expert was a smash hit, and higher-level supplements followed, including the
Set 3: Companion Rules (1984, adding levels 15-25),
Set 4: Master Rules (1985, adding levels 26-36), and
Set 5: Immortal Rules (1986, adding play for characters who had attained Immortality
). The five sets would eventually be collected in the
Rules Cyclopedia, but not before the publication of...
6.
Advanced Dungeons & Dragons: 2nd Edition (1989, aka "2nd-Edition," or "2e"): A revision of the AD&D rules, 2e included the "non-weapon proficiencies" that had been introduced in some of the 1e supplements, kept the 1e ability score tables, but added some classes, removed others, and removed Half-Orcs. Assassins and Monks were removed, Magic-User was renamed "Mage," and the core classes of Bard, "Other specialist Wizard," and "Other Specialist Priest" would be added. Once again, supplements would expand the options of 2e, Kits would be introduced, revised, and re-revised before a new edition would come. But in the meantime, we got...
7.
The Dungeons & Dragons Rules Cyclopedia (1991, aka "RC"): This reprinted and collected the rules from the Basic, Expert, Companion and Master sets, and provided a complete game from levels 1-36 in a hardcover book. There were minimal alterations, although the druid and mystic (kinda like monk) from the Companion and Master sets were added to the earlier offerings of cleric, dwarf, elf, fighter, halfling, and magic-user. The two lines would eventually re-converge after TSR was acquired by Wizards of the Coast and finally released their version of...
8.
Dungeons & Dragons (2000, aka "3rd-Edition" or 3e): This edition merged the disparate lines, codifying and combining all the disparate rules of various editions into a combined system that took a middle road between the unified stat bonuses of one line and the slightly higher bonuses available in 1e/2e to give us the 3 (-4) to 18 (+4) system we all know today. 3e followed the presentation of AD&D but notably dropped the "Advanced" because it was also (arguably) the 3rd major edition of the B/X line. However, it was also the 3rd serious revision to the ruleset, following OD&D/Holmes (1e), AD&D and BECMI/RC (both "2nd editions"). WotC has remained notably silent on which line of inheritance they were claiming when they called it "3e," possibly because the answer is "all of the above."
I could go on, but I think the history of versions from 3e to now (3.5, 4e, Essentials, 5e...) is well-known, and I have places to be.