The REAL and HUGE as well as BASIC innovation in D&D was the following:I really think identifying the Gygaxian Building Blocks (spells, magic items, monsters, special abilities, traps) is one of Sett's great contributions to the understanding what it is we do when we play this thing called D&D. You want a way to make the game your own but still recognizably D&D? Make your own blocks but don't change how they operate/interrelate.
1) Providing radically new building blocks for fictious situations.
2) Providing a robust model for interaction of said building blocks.
3) Providing the idea for interacting building blocks
- magic items
- special abilities
- traps(- planes & gods)
the combat stuff was already there in some form. Just look at the monsters, at the spells and realize how this stuff was basically made from whole cloth!
I cannot emphasize the importance of that enough.
Sure there are conceptual sources. But the procedure in which source material and original ideas were mixed and mashed and formed into interactive building blocks for challenges and their resolution, is creative genius of the highest degree!