What makes Arthurian fantasy its own genre, different from more traditional D&D-ish medieval fantasy? What are some Arthurian-style plots?

[Eyes of Nine]

Big magic with a cost, rare (Merlin can do big magic, but he's moving backward in time)
The Chosen One protagonist (ideally visibly by some supernatural creature)
Human emotions and foibles contrasted with "goodness"
Chivalry

 

[aramis erak]

I'd add the 1998 miniseries Merlin, starring Sam Neill. It takes some liberties, like adding Queen Mab as sister of the Lady of the Lake and antagonist, but Miranda Richardson's portrayal of Mab along with Martin Short's Frik are a great portrayal of malevolent fey.

Never saw it. But, given the cast...
Mab is in several variations and ways. Prior to White, pretty much every version reconsiders and reworks a lot. And they're all anachronistic to a degree, but White's Once and Future King is, for many, unreadably Victorian.

Alternatively, there is the Arthur Dies at the End series of books, a sardonic retelling and critique of Mallory's work, because as seminal as Le Morte d'Arthur may be, by the Grail is it dry.

There is a pretty decent translation that's less dry... it omits the repetitive recitations of lineage. One can tell Sir Thomas was writing for a periodical an paid by the word.

 

[aramis erak]

Big magic with a cost, rare (Merlin can do big magic, but he's moving backward in time)

He's only moving backward in a few versions.