D&D has gone through so many changes since it's inception, that I'm surprised that the 2e and 3e holdouts even call those systems D&D. In the Judges Guild supplement The First Fantasy Campaign the D&D game represented there bears no resemblance to what it would become just a couple of years after. In the original D&D game (1974 version) fighters were called Fighting Men, elves could switch from magic-user to fighter every adventure, clerics could not cast spells at first level and there was no thief. People complained when 4e dropped gnomes, but the original game didn't have gnomes. 2e introduced a point buy system in the Optional rulebooks and a convoluted critical hit system that frequently had players suffering broken bones or severed limbs. In between those rulesets, they added a proficiency system, kits, weapon specialization, honor and had dual rulesets for normal and "advanced" players. Which version of those rulesets is real D&D? And let's not even get started with 3e, which somehow critics were willing to accept as D&D even though it's as completely different from 2e as 4th is.
I think of D&D like a group of children with different fathers. They are all related and I love them equally.