For me, the essence of D&D is expressed in the Foreword to Moldvay Basic:
I was busy rescuing the captured maiden when the dragon showed up. Fifty feet of scaled terror glared down at us with smoldering red eyes. Tendrils of smoke drifted out from between fangs larger than daggers. The dragon blocked the only exit from the cave. . . .
I unwrapped the sword which the mysterious cleric had given me. The sword was golden-tinted steel. Its hilt was set with a rainbow collection of precious gems. I shouted my battle cry and charged.
My charge caught the dragon by surprise. Its titanic jaws snapped shut inches from my face. I swung the golden sword with both arms. The swordblade bit into the dragon's neck and continued through to the other side. With an earth-shaking crash, the dragon dropped dead at my feet. The magic sword had saved my life and ended the reign of the dragon-tyrant. The countryside was freed and I could return a hero.
I've played two versions of D&D mechanics that I felt delivered this essence: Oriental Adventures (the 1986 version),
and 4e (which did a better job than OA).