The Phantom Tollbooth has a little bit of the overdone fantasy element of "good vs evil" where some evil power is conquered at the end by some regular, simple person (dividing stuff into blanket good and evil sides is very annoying to me), but like Voltaire it's kind of satirizing the whole thing. The Phantom Tollbooth is iconoclastic on a kid's level. Most fantasy is actually seeking to reinforce these symbols, rather than challenge them. I know the flippant style of NP types doesn't always sit well with IxxJs (and it's clear Voltaire and Norton Juster are Ne-dom), but it's not just a series of witticisms. It IS painting a bigger picture, but they don't bother to connect the dots for people because the obviousness of that is very dull to an NP. Most fantasy genre hits you over the head with a sledgehammer regarding its theme and any symbolism it contains. These move so quickly from one silly situation to the next that you can easily miss the point each is making.
The Phantom Tollbooth's real themes are about forming gratitude for the world around you and finding personal motivation and inspiration, as well as understanding what is true wisdom vs mere convention, which involves becoming aware of how arbitrary the world can be structured. This is almost opposite of most fantasy, which wants you to believe everything has some meaning, and that there is some fate you are woven into, that you are destined for a course, as opposed to you being a creative force and using your perceptive powers to discern principles, not needing to follow some spelled-out code or being confined to a particular path.
The character in PT is getting a sense of control over his perspective (and as a result, his reality) in every thing he encounters (not stuck in the doldrums - as a state of mind), and he learns he can find reality interesting because of his own creative ability. He sees there are many ways he can interpret everything and that things were only dull because he chose to be dull. He learns to remove assumptions.
At the outset, the lead character has no gratitude and difficulty finding anything to muster energy or enthusiasm for. It struck me as different from most fantasy because it doesn't have the hero themes at all nor is it about powers of good and bad, but it's about creative power and forming powers of discernment. Rhyme and Reason reign at the end - which gives the ability to give meaning to things, not simply joining some black and white side of good or evil. Everything is just a bunch of arbitrary facts without it, which is why the Kingdom of Wisdom is very silly until those two reign again. The main character feels and witnesses extremes of "rhyme" and "reason" throughout the novel, which also emphasizes the balance needed for both. What this book also does, similar to Voltaire but on a children's level, is make fun of the norms people take for granted, to reveal how absurd so much of it is. It's so anti-Si, it's hilarious to me. It's mocking conventional ideas of wisdom, morality, etc. In being "random", it's illustrating how very arbitrary human "order" and rules really are, that they've actually eschewed rhyme and reason.
The fantasy genre otherwise strikes me as an e9 genre - escape from reality in the fantasy of the humble, regular person conquering some big bad force by being diplomatic and humble in working with others, but then developing backbone when needed. The theme is right on the surface and there is not much thought needed to suss it out. Everything I know about Harry Potter fits this.
I liked fairytales as a child, but most are more like fables to me than a full blown fantasy epic. I do admire the work of creating a "whole world", but I don't think that in itself is captivating enough for me to really enjoy the genre.Straddling the line, between Phantom Tollbooth and more *serious* work, we have Narnia, The Chronicles of Prydain, The Artemis Fowl books (magic intermingled with technology), and oddly enough, Toy Story.
All of these books combine the action and plot in a whimsical setting, or at least 'extranatural' if not supernatural...but also include themes such as sacrifice, redemption, moral quandries, perseverance, and good fellowship.
Science fiction, itself, can shade from whiz-bang entertainment stretched over a thin framework of barely-defined philosophy (Star Wars), to rich worlds such as Dune, or James-Bond in space encompassing large-scale sociological commentary such as Dominic Flandry...on to Tolkien or soft-porn such as Gor.
It seems that the more the plot involves fantastical elements, as you said, or quasi-supernatural, the more it deserves the label Fantasy: except when one gets to work such as Gor, (or related works such as Conan The Barbarian or Tarzan) which is an entirely different kind of, ahem, "fantasy" altogether...
The Narnia series has obvious symbolism, but I only liked them for the story. They didn't impact me because the themes felt obvious (very Biblical) and I had no perspective shift from it. I don't include them with my general criticism of fantasy though because I liked them and they are not favorites for reasons unrelated to my criticisms. As with most novels, I skim over very long passages describing action, especially warfare, and there is often too much of that in the fantasy genre for my taste.
I never liked Tolkien's Lord of the Rings, sorry - I know you're a huge fan. I think these books are more social in nature - they read almost politically to me (and I prefer something like Orwell for that). There's this personal quality lacking...perhaps they are not "psychological" enough. Most books I like have very little plot and deal with no supernatural or even extraordinary events (I believe you thought at first that Proust was writing about the lives of the social elite, which is not at all what Proust really wrote about & is a mere backdrop, but you later on saw that), but they are heavy on the exploration of the psychology of the characters & deeply rooted social dynamics, and in the end, they say a lot more to me about human nature and reality. I don't get a deep sense about human motivations in fantasy genre - they seem to espouse moral lessons and vague, general themes about human virtues and vices. Of course, there are varying degrees of talent in any genre, but generally I find fantasy to attract writers who are not as good as those in other genres and don't develop their themes (if any) so deeply. Harry Potter is prototypical for me, then, not any outstanding exception.
Anyhow, my point about this and intuition is that people sometimes think a taste for the fantasy genre is indicative of intuition, becomes sensors must scorn anything not "realistic", but I don't witness any strong correlation there. The genre itself doesn't have a stronger tendency to expose what's behind the concrete layer of reality anymore than other genres (and arguably, some fantasy novels can do so even LESS), even if it has supernatural elements.