Long ago the foundation was laid for Dungeons & Dragons by the old gods of Gygax, Arneson, Kunz, Kask, and others. This foundation has continued to be at the core of the game. I see that core to be:
- Hit Points
- d20 to attack*
Quite simply that is the basic structure if you ask me. With those and those alone you can play a game. Remove those and the core begins to change into something else. Now, I realize that this is a bold statement, and perhaps takes in more games/editions/versions than some are comfortable calling D&D. But I look at it this way: if those elements are present, I am going to play it almost like I always have with little reference to other more obscure or less essential rules. And more importantly play it in the tone or fantasy sub-genre I want.
Classes are the basic feature of our games that really define the way we play. They can be as complex, simple, well defined or blurred as you like, but classes are an essential part of D&D identity.
Levels are also an aspect of play that is closely connected to classes. Without levels much of the metapurpose of D&D design is lost.
HP, initiative and the d20 to attack are all defining componenets of D&D combat, that if lost or substituted would sacrifice the nature of play.
And lastly, the effect of saving vs the many noncombat encounters are what I consider the core resolution system of non-combat encounters. One could argue that ability checks or skills or the like are essential, but I don't feel this is the case--even though I someitmes use them. I can flow with D&D with or without these components, but saves are essential to me.
So what does this imply? Simply put it implies lots of games are okay by me. With these structures in place I can play them with my preferred rules lite, fast and flexible style of play in my gritty, somewhat dark and deadly flavor of a fantasy world.