Gandalf should've stayed dead in Lord of the Rings
One of the times I have to agree with G.R.R.Martin on Lord of the Rings (do not bring up the tax policy interview).
The Lord of the Rings, in part, can be viewed as a coming-of-age story. On a personal level, for the Hobbit characters as they go through what even Tolkien would agree is deliberately similar to the experience of many young British men in WW1. But also, as a larger picture, mankind.
In the Legendarium, God and his lesser godlings play less and less of a roll in the day to day of Middle Earth as time goes on. Early on in the first age, they directly interacted with mortals and those beings living in Middle Earth. One Maiar (Angel equivalent, same order of beings as Gandalf) even marries an Elf. Over time, this drops. After the War of Wrath that ended the first era, wherein the forces of essentially heaven battle the forces of evil in a war so destructive it sinks a contintent, the only other direct divine intervention comes near the end of the second era thousands of years later, when the Numenorians, advanced, divinely gifted but eventually supremely arrogant, colonialiast, mortality fearing humans, try to invade heaven on Earth, and he this time deliberately sinks their continent.
After that, the only role the lesser godlings and angels of the setting play is five Maiar, in the guise of wizards, are sent to Middle Earth to forment the resistance against Sauron, the new ultimate evil in the world who is slowly regaining their power. They are supposed to be a more supporting role, serving as advisors, guardians of the wild life, teachers, not rulers or warriors.
This theme of the world becoming less and less magical, or really less and less divine, is key to Tolkien's work. The whole eventual end point of the book is that, after the defeat of Sauron, the world is left entirely to Mankind. The Elves leave or fade away into nothing, the Dwarves go extinct, the Ents stop moving and turn into regular trees, and Aragorn wipes out the Orcs. The only direct intervention in the entire story of LoTR explicitly said by Tolkien or referenced by the characters is at the point when nothing else can secure victory for the free people of Middle-earth. Golum, having claimed the Ring at Mount Doom, where nobody had the willpower to resist it. Eru-Illuvatar makes Gollum trip and fall into the volcano.
Anyway, this theme is somewhat reduced by Gandalf being the leader of the fellowship. Yes, sure, everyone swears to Frodo, but its Gandalf advice that is always taken, always the correct choice. Don't take the ring through the Pass of Rohan to Gondor. Don't sail around. Can't take the eagles. Is this really a story about mankind stepping up to the plate, taking the world God is leaving for them, if an angel is there every step of the way telling them what to do? Admittedly, Gandalf agrees to let Frodo change his mind and go through Moria when the pass through the mountains proves untenable, but that's the only time.
But hey! Gandalf dies. Suddenly, the Fellowship, and by extension the mortal people of Middle-earth, are on their own. They must fight their own battles from here on out. They descend into infighting and split up to cover different ground; the fellowship is shattered. An interesting way to take the story, get rid of the Deus Ex Machina character from The Hobbit and raise the stakes in a way that fits the theme that Tolkien is setting up.
Except then Tolkien immediately brings him back. Gandalf is the one who cures Theoden of his poisoning, the one who brings Eomer and his riders to the Hornburg, he's the one who takes command of the siege of Minas Tirith when Denethor is incapacitated. Still, mankind is not really entrusted to fight for itself; it still has God there directing things.
Obviously, Tolkien was very Christian, so this isn't exactly surprising, but I think this is where his values and his decision to bring back Gandalf directly conflict with the story he's trying to tell. This is even more of a problem in the movies, where Denethor is even stupider and Gandalf has to do things like arrange for the beacon of Minas Tirith to be lit. Or, not directly tied to Gandalf but is tied to more deus ex machina's working against the theme of the story, the magical ghost army completely curbstomping at Pelenor Fields when they only scare the Corsairs in the books.
I just think this is one Tolkien dropped the ball on. Tolkien heads will tell you that Gandalf is totally changed by his resurrection, which personally I don't think is the case, and that no there still were dramatic stakes because Gandalf was out of the story for, in-universe, about two weeks. But I think the story works just as well if, say, Aragorn was the one to heal Theoden (Hands of a Healer being the Hands of a King foreshadowing), Eomer or one of the other two hunters come to the Hornburg on their own, and if a normal mortal man, perhaps Faramir or even Pippin, take command of the siege of Minas Tirith. If the story is about Man claiming their role as the guardians of the world, then let them do so.