Stormcrow posted something I pretty much agree with: that the essentials of D&D are really the stripped-down elements of character description and a few concepts linked to those elements. Most of what we think of as "D&D" could be ignored by the player and supplied by the GM -- and house-ruled in some pretty extreme ways. For example, the combat system, including armor class, is not important, since it can be completely replaced (and has, with ascending vs. descending AC, Chainmail combat, "alternative system" combat, people substituting Metagaming's Melée combat system...) Everything other than a few basics can be ruled by the GM as necessary.
We agree that these components are essential:
- The six abilities (although honestly, I think only the idea of the abilities is necessary; the actual abilities can change.)
- Class, and some kind of advancement system.
- Some way of telling how long you are likely to survive combat ("hits".)
- Gold Pieces, and stuff that can be bought with them.
What do you think could be completely replaced in the game, but still leave the game as essentially D&D?