Our first jump in between movies is big. From 1940's Pinocchio we're moving all the way to 1963's The Sword in the Stone, which is a much bigger jump than when I went from 1937's Snow White and the Seven Dwarfs to 1950's Cinderella. I wished at first that I had a movie to represent the 50's, but it turns out that The Sword in the Stone was close enough to that era to capture some of its essence, after all. In fact, there are definitely some similarities between Arthur and Cinderella--and between Arthur and 1959's Aurora/Sleeping Beauty.
Like Pinocchio, Arthur is primarily just a boy. Interesting to note that while the Disney princesses are mostly teenagers (that is, young women who could easily be anywhere in the range of fifteen to twenty-five), the male side of films starts off with younger children. Pinocchio was about, what, eight or nine at the most? Arthur is around twelve, and physically he's on the young side, still thin and coming into his own. A child, not quite a young man yet.
When I said that he's primarily just a boy, I meant his personality, too. Not entirely unlike Cinderella, he's defined more by certain things that he says or does than by a general sense of who he is or what he would choose to spend his time doing. That is, his personality is just that he's good. He takes to Merlin's guidance right away because he is perfectly willing to submit to a higher, positive authority. And when Merlin uses his magic to do the kitchen duties, Arthur begins to protest by telling him, "I'm supposed to do it." So he's honest--and even though Sir Ector and Kay don't treat him well, he spends absolutely no time complaining that this isn't fair. He just does his duties and looks forward to possibilities (becoming a squire).
In this way, he is completely like Cinderella, except perhaps more practical and less dramatic. He even sings while he's cleaning (a pot in the kitchen instead of the foyer floor). I guess, while Cinderella has lived a different life, this life is all Arthur knows. All he knows is being a servant, so to him there is nothing shameful or negative about looking forward to being a squire (true that Cinderella doesn't even have a higher level of servitude to look forward to). He doesn't need to dream about far off castles because, like he tells Merlin, he doesn't "have any problems."
Not to say that Arthur is completely subservient. Because he is honest and good, he recognizes Merlin as good right away and is even willing to stand up to Ector on Merlin's behalf. He doesn't ever attempt to stand up for himself (I don't think the thought even occurs to him), but he is quick to defend Merlin.
I made the comparison between Arthur and Aurora because both of their stories contain this concept of royalty that cannot be hidden. Aurora is regal despite being raised as a peasant girl in the middle of the woods. And Arthur is the king in the guise of a servant. Both of them have a hidden identity, a secret that can't be contained by mere physical traits/surroundings.
The biggest difference between Arthur and any of the princesses is surprisingly (for me, at least) hard to admit, though it's so big. The Sword in the Stone is all about education, something that isn't really emphasized for any of the princesses, though possibly in part because education doesn't always make for the most interesting story. Think about it--whose favorite Disney movie is The Sword in the Stone? I'm sure a couple of people would name it, but in general it isn't one of the top films. (And I'm not counting Belle's story as being about education. Yes, Belle liked to read and her conversation with Gaston, who was disgusted by the idea of a woman getting ideas and thinking based on her reading, did veer toward the concept of education. But in general Belle's reading is more her passion and her escape than a quest for knowledge and "bettering herself.")
Right away, Merlin comes in and looks at what Arthur has been studying (the "sporty" side, with jousting and swordsmanship and the like) and says that what he needs is an education. So Merlin, through these various trips turning into animals, teaches Arthur about different perspectives and about how all of the pieces can come together in a situation. He's constantly emphasizing that "knowledge and wisdom is the real power." Whereas for the princesses, goodness is most important, Merlin believes that Arthur will act out goodness if he has a foundation in knowledge.
So while Arthur might not have the most exciting film of them all (if you're watching the two of them turn into fish and birds and wondering when you finally get to the sword in the stone bit), his story does emphasize positive traits, including that theme of education that none of the others really stress. Honesty, diligence, and the ability to listen and learn make up Arthur's character.