At its inception, D&D is a medieval fantasy game. Yes it pulled from different literary sources. But it was firmly rooted in medieval fantasy with sword and sorcery tropes thrown in for flavor.
This, combined with Sword and sorcery books, and Tolkien's strong parallel influence on the fantasy genre at the time has influenced what many see in their minds eye when they think of the word "fantasy".
I think this is pretty questionable - Gygax seemed to be primarily influenced by S&S with medieval fantasy thrown in, if anything. Perhaps Arneson was the other way around, but looking at early D&D materials, I see a lot more Leiber, Vance, Moorcock and so on and a lot less Tolkien, outside the basic races, which Gygax has made some contradictory claims about, at one point indicating they were basically forced on him by players. I see very little of medieval myth and lore in there. Not none, but it seems pretty secondary to a strong S&S vibe.
Indeed, if anything, it seems like AD&D became more medieval and less S&S over time. Specifically, 2E's core books seem far more "medieval fantasy" than 1E does, as does the Grey Box FR as compared to Greyhawk (which is all over the place), if you compare the books (I started playing in 1989, so just as 2E came in). A course which then more or less immediately abandoned in favour of wilder realms - the first setting, Taladas, was basically early dark ages (say, 500AD) Eurasia with some fantasy lands and a bit of the Pacific Islands thrown in for flavour. Spelljammer, Dark Sun, Planescape and so on all contained basically zero medieval fantasy.
However, I think we can both agree that it's some formulation of S&S, Tolkien and medieval fantasy.
I and I agree with:
But for lot of people newer to the game their first exposures to "fantasy" are not Medieval folklore and myth, Tolkien, Howard, or sword and sorcery anything.
As these generations start to interact we now see a wide variance in their aesthetic preferences when it comes to D&D game world visuals.
I mean, I'm 42, and my first exposure to fantasy was basically Greek/Norse myths read to me as a child, and then my first contacts with "proper fantasy"-style stuff were largely Disney via things like the Sword in the Stone, The Black Cauldron, and yes, Gummi Bears. Then A Wizard of Earthsea, which I think, more than any other fantasy novel, is burned into my soul, tonally and aesthetically. And then as I got into wargaming and D&D we have a huge rush of influences, with both D&D settings as themselves primary influences (i.e. before I'd read much of the fantasy that inspired them), and stuff like Warhammer (which itself is derived from many of the same works and even to some extent from D&D). So I guess I'm around the first generation where you start getting "circular" influences, where D&D is part of the circle.
Personally, I love a wide variety of styles. The only one I don't like for D&D is "comic book" (which 1E does a bit), because it just doesn't feel right. Even straight-up Disney feels more correct to me than '70s/80s comic-book style stuff.
I'm a sucker for some pretty severely low-key and romantic/medieval stuff too, so long as it doesn't getting too cutesy. Unfortunately the Dolmenwood stuff above exceeds my cutesy limit. Conceptually, I like it, and if you described any of the pictures to me with words, I'd probably think I'd like them, but in practice it's just a little bit too affected, too conscious in it's faux-naivety. So I see it as clever and well-executed but it lacks any emotional connection.
Maybe I just need practical-looking adventurers to ground my fantasy.
