Well, over years of rule-changes that incorporate all sorts of additions, deletions, and reimaginings (particularly with 4e's combat powers and classes) what provides the island of stability for the game? What provides the continuity?
In the shift from 2e to 3e, as substantial as the rule changes were, it was the fluff. And I think that became such an important element of people's comments because the changes in the mechanics were so far and wide.
But what fluff? The core setting was Greyhawk. But there were tons of people that played FR or Eberron instead. The fluff there is different, sometimes even considerably.
The fluff is also very different from older settings, say Dark Sun or Dragonlance.
It all leads back to the question of "what is D&D", of course. And again I say it's not something that we can define entirely objectively. There is a subjective component, and if you played only AD&D with Spelljammer you would have a hard time recognizing D&D 3E or D&D 4E - in its core - as "your D&D", even if you would still see the similarities and many common elements. The focus of the game would seem very different, since there is no Spelljamming by default (well, at least D&D 4 Adventurers Vault gives you the necessary vessel back), and instead you typically operate on one world and explore the dungeons and fight dragons there.
The "continuity" is stuff that is cited again and again:
- Rule concepts like classes, hit points, XP or the distinction between magic types (arcane, divine as a "traditional" distinction)
- Classes like Fighter, Wizard, Cleric and possibly Rogue
- Races like Elves, Humans, Halflings, Dwarves
- Spellcaster Names in Spells (Bigby, Leomund, Tenser)
- Monsters like (color-coded) Dragons, Mind Flayers or Beholders
- Parties fighting against monsters in a dungeon as a "typical" adventuring scenario.
- Magical Items like Flaming Longsword +3
I am not sure if everything of this has _always_ been in D&D, including OD&D. But that doesn't matter, because a continuity is not one state, but a sequence of related states.