Complex, Learned, Engaging
The Sword in the Stone is usually considered a great work of YA fiction (even if the other three books in the quartet get really adult really fast), but don't think White's going to talk down to the kiddos. The complete opposite is true: his style is complex and learned, but it's also highly engaging.
One thing that adds to White's complex style is the almost continual use of technical jargon. This guy has really boned up on his vocabulary relating to virtually every art and craft and pastime of the medieval period. So, the books are chock full of words that are specific to falconry, jousting, castle architecture, plant life, medieval hunting, and even cricket (yes, cricket).
White also spends a lot of time making many of the characters speak with regional, ethnic, or class-inflected accents. Some of these can be difficult to read, like Gawain's Scottish:
"Ye have done more than e'er a body could expect," he said warmly, holding the veined hand in his paw, "and now we must look onward for the best. Let my brothers go unarmed. He willna hurt them, gin he see their faces. I maun stay ben with you." (C.8.87)
Translated, the gist is: "You have done more than ever anybody could expect [...] And now we must look forward for the best. Let my brothers go unarmed. He will not hurt them, if he can see their faces. I must stay inside with you."
As you can see, trying to understand Gawain's accent (and those of Meliagrance, Little John, the Sergeant-At-Arms, and others) can be challenging and slow the reader down a tad.
He's a Bit of a Show-Off
White also likes to show off his academic chops, which contributes to a highly learned (say it with us the English way, with two syllables) style. He knows a lot about medieval history and culture, and he's not afraid to demonstrate it—in exhaustive detail.
Check out, for example, The Ill-Made Knight, Chapter 8. It probably has the highest frequency of name-dropping in the entire four books. You really need to have the interwebz handy while you're reading this, so you can look up figures like Gregory of Tours, Lully, the Knight of the Tower Landry, and Duruy.
We'll cut White a break, though, in showing off his mad research skillz, because it really helps to immerse the reader in the culture and time period.
It's Like We're There
That's not to say that this complex and learned style is dull and boring. Far from it. Instead, it's super engaging, and gives the text an immersive feel. All of the details have the effect of making us feel like we're part of the action, since it's so vivid.
Another aspect of the author's style that makes us feel like we're there is his use of the second person: "you." So, when the narrator walks us through the Castle of the Forest Sauvage, telling the reader, "Once you were inside the curtain wall..." you catch a glimpse of this and that (S.5.4) or "If you look down and are not frightened by heights," (S.5.5) there's a feeling of immediacy—like we're right there being led through the castle (and in a way, we are).
This second-person engaging style also has the effect of making us empathize more with certain characters. Toward the end of The Ill-Made Knight, the narrator invites us to consider certain questions:
"Do you think it would be fine to be the best knight in the world? Then, then, also, how you would have to defend the title." (K.45.10)
White is, of course, talking about Lancelot. And by asking us directly to consider the knight's plight, the style becomes more immersive and engaging.