My friend Brian (@Deathtrap Games on Twitter) shares some thoughts on what is D&D, really, over at the Grumpy Wizard blog:
A while back I was binge reading Grumpy Wizard when I saw an article that really caught my imagination. He described how neither Gary Gygax nor Dave Arneson had actually played Dungeons & Dragons while developing the system. They had both played their own free-form games with increasingly open and evolving role playing elements; Their own respective Formulae of Chainmail mixed with Braunstein mixed with Stratego. Dungeons & Dragons was, at its inception, a mutant third way between their ideas.
Check it out, it’s worth a read: There is No Dungeons and Dragons