Watch it or do I get to say the same about Eberron, Planescape and Spelljammer. I dislike all three, but a) I am not going to claim they are not D&D; and b) Darksun is just as much D&D as those three settings.
I said Dark Sun is the least D&D you can make D&D, not that it was a bad setting. I have a fondness for Dark Sun, and mostly because of its differences. But the entire tone of a Dark Sun game is very different than most other D&D games. Heck, there's only one dragon so it can't even really be called Dungeons & Dragon
s.
Still, campaign settings are an iffy bar for what is or is not D&D. As you can play Dark Sun using FantasyAge or FATE or GURPS it's still Dark Sun. Settings are pretty much system neutral.
I agree that D&D is more than just the D20 rules, and is more than just a fantasy world with elves and dwarves (to be clear, I never said that those things were the only requirements for something to be D&D, and you didn't explicitly say that I said that. Although, it does feel implied, but that might just be the lack of visual and audible cues that makes internet communication so iffy). My definition is broad, but it's not so broad that just any fantasy setting qualifies.
D&D is funky. It's equal parts a generic ruleset for fantasy roleplaying and a setting and cosmology. It balances between both more than any other RPG.
What D&D is will vary between campaigns. Sometimes it can even vary between session and session.
I've thought about this long and hard and there's not really a good definition for what defines and limits D&D.
It's not anything with the name, because then you include board games and video games. It's not the ruleset because then you include Star Wars and Gamma World. It's not the IP because then you include the television series and licensed properties. As mentioned above, it's not the worlds as those are settings and can be played with any rules.
At best, D&D is the point where all those things intersect. The nexus of ideas and concepts.
Still... classes and being "D&D enough". That's tricky.
Obviously, you could make anything into a D&D class if you wanted. A shapeshifter, a truenamer, an alchemist. Or a gunslinger. Pathfinder did that one. But is a gunslinger D&D? Does it fit what the majority of people consider D&D and would use in their games? That's the tricky part.
I'm a Ravenloft fanboy. I can handle a gunslinger. Black powder weapons are found in the Mists. But how about other worlds, other games? Is the content getting the maximum play? Is it worth the investment of pages and time to design and playtest? Can it be used in Adventurer's League??