(Using JohnSnow's List) In 1974:
1. Medieval Wargames Campaigns
2. dice pools or 2d6 for combat resolution ...
... unless one used the "Alternate" Matrices
(which most people did)
3. strength, intelligence, wisdom, dexterity, constitution, charisma
4. class levels
5. Human, Elf, Dwarf, Hobbit and Cleric, Fighting Man, Magic-user
6. the magic system (approximately)
But those are only "the rules" at a particular level -- and players were instructed in 1974, "If your referee has made changes in the rules and/or tables, simply note them in pencil (for who who knows when some flux of the cosmos will make things shift once again!)".
Although it was explicitly intended that individual referees should change things at that level, and all in different ways -- "for everything herein is fantastic, and the best way is to decide how you would like it to be, and then make it just that way!" -- there was value in a single common "baseline" referent of the term Dungeons & Dragons. The value was not just heuristic but commercial, and TSR took steps to protect its ownership.
Dave Hargrave, for instance, got a letter from lawyers and deleted the "D&D" references from his supplement The Arduin Grimoire. However, not only did he keep selling his first "trilogy" but he went on to publish more volumes and related products. For all the "gonzo" directions they took the game, part of their value lay in being able to refer back to the lingua franca that was D&D.
What worked for Arduin, with all its variations (guns; d% critical hits; additional attributes; new and revised classes, and different XP and HP systems; saurigs, phraints, deodanths, kobbits; spell points; etc.) naturally worked even more easily with Basic, Advanced, Expert, Companion, and Master D&D materials.
That was the real value of the trademark, the indication that one could pick up a module or supplement and have the Common Tongue of Hit Dice and Armor Class, of an Enchanter casting Hallucinatory Terrain and Massmorph, of a brush with a Cockatrice calling for a "save vs. stone or petrified", and so on.
Now, it's the Tower of Babel. Without being able to look it up, I can't make head or tail of what people are talking about. They might as well be playing Exalted, for all the sense it makes to me -- or talk of my game would make to them.
That's the test that matters at the practical level of buying or not buying Module X or Magazine Y or Supplement Z. Whatever house rules someone might be using at home, what reference do I need easily to make sense and good use of this?
If I need to care enough to know that it's supposedly "Edition N" in the first place, then that's nothing but a pain in the neck. I in fact have never found that necessary when dealing with any TSR edition.
It's like Diplomacy: nice to settle on one edition of the rulebook if it comes down to very subtle rules-lawyering, but the standard board hasn't changed in decades. Fleets and armies, moves and convoys and supports, the game has remained the same in all essentials for half a century. The Variant Bank lists some hundreds of different games, all referring back to that common frame of reference.
