Note that I'm a fan of both series. I love both, I've read both series to my kids multiple times. So I'm neither unfamiliar with them, nor prejudiced in favor of one or the other.
Laura Miller (the author of the article -- other than that I have no idea who she is) correctly identifies that the authors Baum and Lewis are not only separated by at least a generation, but much more by their intent and design in writing. She then, for bizarre reasons known only to herself, decides that despite this she should choose to compare them on the exact same basis (one in which she clearly favors Lewis and uses his approach as the benchmark). Naturally Baum falls short when judged by whether he achieves the same kind of things Lewis does.
On the other hand, she fails to study the Oz books as such, and misses a great deal in that way.
For instance, she accepts rather unquestioningly Baum's own statement about how he was removing all the horror, etc., from the "modern fairy tale" he was writing, and points to a few things that she feels confirms it. Yet I can't imagine how one could POSSIBLY read The Wizard of Oz and see no horror in it. There's some very dark pieces even in that first book -- there is death, dismemberment, threats of hideous danger to multiple members of the party. There are truly grotesque pieces in the Wizard of Oz, and other subtleties that one misses until one's an adult.
As an example of the latter, in both the movie and the book the Wizard is unquestionably a humbug; he hasn't a lick of magic in the first book. But his solutions to the three other questers -- the Scarecrow, the Tin Woodman, and the Cowardly Lion -- are much more slyly humorous than the rather straightforward and transparent ploys shown in the movie (which, I should add, I enjoy as well). Most iconoclastic is that the Cowardly Lion is given a potion which is "liquid courage". The description of the taste makes it fairly clear that what the Wizard's probably doing is simply giving the lion a heavy shot of booze, aka "Dutch Courage" in those days.
In addition, the subtle growth and indications of the true nature of Dorothy's companions is much better done in the book. We can tell early on that the Scarecrow is actually very clever, the Tin Woodman very kind-hearted, and the Cowardly Lion desperately courageous -- and this we learn from events and actions. No one has to tell us this. In many ways, the characters of the book, with their declaiming of their own characteristics, are as misleading as their author; both of them tell us things that turn out not to be exactly true in the book.
Certainly I would agree with Ms. Miller that the Wicked Witch herself comes off more threatening in the movie. But other than that the movie is a failure in one of the most absolutely crucial areas of all.
In the movie, as most people know, it turns out that Dorothy's adventures in Oz were a dream -- a concussion-induced play of her subconscious which worked out a lot of the conflicts that led to her running away. The theme and lesson is to be satisfied with what you have rather than wasting time wishing for some magical miracle to take you off somewhere better.
In the book, Dorothy's adventures are all completely real. She vanishes from the Mortal World for months -- long enough for Uncle Henry to have recovered, found a way to get the money to get the materials, and rebuilt the house -- only to reappear on the lawn in front of the house after the Silver Slippers take her there. She does this after courageously travelling through dangerous and bewildering adventures and gathering aid from many of those along the way, doing so through her own determination and her forthright nature which disarms the suspicions of others and convinces them to trust her, even to take a risk to travel with her. Here, the theme and lesson is to be true to yourself, to persevere through dangers and difficulties, and be willing to trust in others, and others will trust in you and you shall eventually succeed. Dorothy is an ACTIVE participant in the events of The Wizard Of Oz, and its sequels -- especially when compared to the typical female character of the era.
Contrast this with Narnia, for a moment. The principal lesson in Narnia -- being, as it is, a deliberately constructed Christian work, with The Lion, the Witch, and The Wardrobe being a tale that leads up to the parallel of the Crucifixion and Resurrection -- is that while Aslan/Jesus wants you to do your best, in the end your success or failure depends on having faith in Aslan and willingness to submit to his commandments.
I would submit that the latter is, perhaps, a less desirable lesson to teach one's children, at least from many people's point of view. Even if I assume God exists, I'd like to think that he's not interested in my having faith per se, and more in my doing my job for my family and friends as best I can.
Considering both works as a series, it is more clear that there is a tremendously wide gulf between them -- not just literarily (however one wishes to judge that) but -- much more importantly -- in purpose and in approach. Narnia was a hobby for Lewis, one to be used as a Christian teaching tool perhaps but mostly to amuse his children and perhaps those of others. Oz was a business for Baum, the Harry Potter of its day, and it became both more complex and less so at the same time.
In all the books there remain various bizarre, frightening, and horrifying elements, sometimes all the more horrible for their juxtaposition with mundanity; Princess Langwidere and her cabinets full of heads that she could swap on and off like hats (and her casual desire to take Dorothy's head for her collection); the cheerful and jolly Nome King, transforming all who entered his realm into voiceless toys for his amusement; the invasion and sacking of Pingaree, leaving only Prince Inga behind; the story of exactly what happened to Princess Ozma at the hands of the Wizard; the horrific non-death of King Kynd of Jinxland; and many others.
At the same time, Baum's personal beliefs began to influence the books in other ways; people were not immortal in the early Oz novels, but by the time we reach the Emerald City of Oz, the transformation of Oz into the Perfect Fairyland is nearly complete -- and done as what these days is called a "Retcon". It's not consistent and it makes it difficult, sometimes, to reconcile events and choices in early books with the later ones. I actually was able to make some use of that fact when I wrote my (probably never to be published) Oz novel Polychrome. This change reflected (If I recall correctly) a shift to strong socialist beliefs and thus Ozma becomes a sort of perfect conduit for social justice, a fairy ruler who makes sure that everyone gets what they need and who tries to maintain perfect justice and order in the Wonderful Land of Oz.
It is also fairly obvious that Baum knew, at least subconsciously, just how tight a corner he was painting himself into. He originally wrote the ending of The Emerald City of Oz to end the series, seal off Oz from the rest of the world (just as Arthur Conan Doyle killed off Holmes in "The Final Problem"). Like Doyle, he found the complaints of his public and the lure of the money required that he dust off the series and resurrect it. At that point he had to keep finding ways to make trouble in a land that he'd explicitly almost defined trouble OUT of, and in order to do that had to keep inventing either new lands or people who were willing, and able, to violate Ozma's orders, at least so long as they weren't caught.
In no way am I trying to argue that the Oz books are without flaw, as should be obvious from the above. Baum's biggest flaw as a writer per se was that he often would be writing travelogues, not novels with a plot. This actually works for many children -- showing me New and Neat stuff can be enough to draw me in as a kid. But such tales are much weaker. The strongest of the later books are probably The Lost Princess of Oz, Rinkitink in Oz, and Glinda of Oz -- all three of which have fairly strong if simple plots driving them, and whose travelogue elements are firmly in service of those plots.
My main thrust here is more that Oz and Narnia are such utterly different works that they shouldn't BE compared in most ways, other than simply as a point of contrast. Neither work is in almost any way attempting to achieve anything like the other -- with the obvious singular exception that both are trying to entertain children. Both contain a huge amount of inventiveness and effort on the part of their creators, but the priorities of those efforts are very different, as are the specific targets of the inventiveness. Lewis' hero-children leave the world and return with nary a lost moment; they pay no price for their travel, adventure, or enlightenment, and indeed their memories of the other world fade at least in part (and apparently, if they stay long enough, so do memories of this world fade, as we see in the sequence with the grown-up Pevensies as the Kings and Queens). Baum's hero-children are gone from our world for exactly as long as their adventures take. Parents or guardians are traumatized by their loss, the children who return must deal with the loss of time. With little exception, the power of Narnia has no effect in the real world, and the children are heavily discouraged from attempting to use any influence or power they may have in that direction; only Aslan may offer such things, and he will not do so often. The power of Oz is weakened in the Mortal World, but far from powerless, and in some ways can indeed work directly even there, and the child-heroes can use that power if they wish; Dorothy, for instance, refuses to move to Oz unless her aunt and uncle come with her, and this demand is granted. So, too, is another difference evident; Narnia is meant, at least for the visiting "sons of Adam and daughters of Eve" as a spiritual training ground. Once you've learned enough, once you've grown old enough, you can't come back (until the end of the world/you die). Oz is... very exclusive, in the sense that it doesn't want thousands of immigrants, but it is not closed to heroes. If you come to Oz and want to stay, you are welcome to do so. You may, like Dorothy, even be able to get a couple of other friends or relatives to come with you. Oz isn't anyone's training ground; it's simply a very different place with very different rules.
Ah well, enough ranting for tonight, it's getting late and my fingers hurt.