The Invention of the GamemasterI personally feel that the greatest contribution made by Gary Gygax et al is the *idea* of tabletop role-playing, and not specifically the core structure of the D&D ruleset itself. Which, if we're being honest, is pretty awkward and clunky. So was the Model T, but that doesn't mean Henry Ford wasn't a frickin' genius.
And IMO, what makes tabletop role-playing such a brilliant new idea is the concept of having a gamemaster. Chess doesn't have a GM. You don't get to say, "this Rook has decided to set off to see the world - what's in that next square past the edge of the board?" or "what kind of terrain in in this square of the chess board, and can I use it for cover to get the jump on that enemy Knight?". There's nobody to ask, and nobody empowered to make up a binding answer. GMs turn what would otherwise be a (possibly very engaging but) strictly limited strategic exercise into an open-ended, endlessly fascinating *adventure*.