I've started reading this big thing, and so far (nearly done with Book One of about a zillion) it's way more fun than I expected! Two things jump out at me:
1. Arthur is presented as a hothead, with Merlin showing up and finger-wagging him into behaving more wisely than Arthur is inclined to do. Merlin plays a role not unlike that of many biblical prophets. Also, much of Merlin's wisdom seems like the kind of thing a hotheaded young man needs to hear: don't seek revenge against a potential valuable ally, etc.
2. Like all fantasy nerds, I've always though that one essential characteristic of archaic language is a certain stylized flowerness. E. R. Eddison's The Worm Ouroborus, the King James Bible and Shakespeare have a lot to answer for in this regard, although they do it well to wonderfully. Malory puts paid to this notion, with a prose style that is archaic but as simple and direct as Hemingway. The clarity and terseness of his prose stays out of the story's way.