I think so. @Umbran 's suggestion that we're really asking about "sacred cows" is on point.
You can tweak how the rules work, but since the beginning, a character sheet for D&D has THE 6 ability scores, your race & class, AC, hit points, and saving throws. No matter how the rules have been tweaked since (e.g THAC0 vs. d20 plus), it's always come down to a core appearance that hasn't changed.
I (vaguely) remember switching from OD&D and thinking how big an upgrade it was that my dwarf could be something other than a fighter. So I really stretched and wrote up his nephew that was a fighter/thief.
In comparison to other systems I've played, IMHO DnD is at its core defined by a class based leveling system as opposed to a more gradual or open system like Shadow Run and Vampire (at least the older versions I've played). Even when I've played Starwars, Pathfinder and even Spy Craft they felt very grounded in a DnDish reality.