Its Elves and Dwarves going into a dungeon to kill things with arrows and hammers. Really.
I don't mean this directed at you solely, Jools, but this is an example of a definition that:
1. Is trite and unhelpful.
2. Overly broad (other games do this as well).
3. Is also overly narrow (many D&D games may never have an elf, a dwarf or a dungeon).
Sorry, again, I'm not trying to direct this at you or anyone else specific. But when discussions like this come up and someone chimes in "D&D is rolling a d20" all it seems to do is obfuscate any attempts to find an actually helpful definition.
(That was going to be the end of my post, but...)
WAIT A SECOND.
What I just typed made me think a bit more. While I stand by my statement that definitions like that are generally not helpful (to the community as a whole's understanding), I need to stand back and take your statement with a bit more consideration.
Maybe, to you, that is a GREAT definition.
While it doesn't "define" all that encapsulates D&D, and nor does it exclude things that aren't D&D it gives me a lot of information about how you play, and what other games you may or may not have played. I'll assume you haven't played other games with dwarves and elves dungeoncrawling. I'll assume you like dungeons, and also the standard races (may or may not like politics driven adventures and the nonstandard races like tiefling and dragonborn). More importantly, it shows me what the really important part, the defining core, of D&D is to you.
Because, in part, we're discussing what definitions of "D&D" are good enough for each of us. Someone earlier (in another thread, or in this one?) had an excellent post about music and the different perspectives she and her ex-boyfriend had because of the style and focus they each had.
So, in a similar vein, maybe "overbroad" is unfair to you. Maybe, for it to feel like D&D, all it really needs is dwarves, elves, and dungeons. Had they, instead of holding off on gnomes in the 4e PHB, held off on dwarves, that might have been enough for you to feel as though it "wasn't D&D" until the dwarf race came out.
So I guess what I'm saying that I might be being unfair to you by saying your definition is flawed. It's not useful to me, but that doesn't mean it's not useful to you.
For me, and I'm starting to understand why pemerton is having so much trouble coming up with a satisfying "true" definition as I try to give some explanation myself what is necessary and sufficient for "D&D". I don't think I can do it.
But in the end, I think your post was actually very valuable for understanding that the claim of "it's all D&D" can't be supported as a universal claim. To some, that may be true, to others it is clearly not. A 5e that has everything in 4e D&D (or 3e D&D) except dwarves, elves, and dungeons would not be D&D to you.
No amount of "let's just all agree it's the same general concept and game we enjoy" would make it fit with your concept of what matters and forms the core, the essence of D&D to you.
Cheers.