First let me start by saying that I am a HUGE fan of the trilogy. I own the extended version, and I have watched it a half dozen times. I think it's one of the best movie series ever made.
That being said, allow me to write something about the logical problems in the story of the movies. I am going to write about the movies rather than the books, as the last time I read the books was something like 15 years ago and I remember next to nothing about them. Many of these problems may be identical to what is in the books, but some might be specific to the movies themselves.
Thus everything I write is from the pure perspective of what the movies depict. The books go into much greater detail on many things, but I am not writing from that perspective.
The movies don't explain too profoundly what the Rings of Power are, except for the speech at the beginning of the first film. If I had to deduce what they were trying to say, it would be something like this:
A very long time before the movie settings, Sauron, some kind of extremely powerful being, made some Rings of Power (what those powers are is never specified) and probably acting as a benevolent being gave these rings to the most prominent leaders of elves, dwarves and men, which were the three dominant species of mortal beings on Middle Earth at the time.
However, Sauron was not benevolent at all, but completely evil, and his plan was to dominate the three species by controlling their leaders through these rings. To do this he created a "master ring", "The One Ring" which he could use to dominate the others. The movies don't explain at all why it was necessary to create such a ring and why Sauron couldn't simply dominate the other rings directly, but whatever.
The movies also don't explain at all what happened then. Were these dominant species dominated or not? Apparently the plan failed at least at some point because none of the three species were under the dominion of Sauron. On the contrary, at some point men and elves raised against Sauron and defeated him and his forces in battle.
So it seems that Sauron's plan was an utter failure. And in the process he made himself extremely vulnerable and mortal, but I'll dig deeper into that detail later. One would think that such a powerful being would be smart enough to not to fail in such a catastrophical way, but whatever.
The movies reveal what happened to the nine kings of men who were given nine of the rings: They were completely possessed by evil and converted into immortal demonic creatures. For some reason, at least at some point in history, they were still not able to rule the species of men under Sauron's control.
The movies make absolutely no mention about what happened to the dwarf leaders who were given the rings.
The movies do reveal what happened to at least one elf who was given a ring: She still has it, even after thousands of years, and probably uses it.
The movies do not dig any deeper into this. It doesn't seem to make too much sense. Surely this elf knows that the ring is evil and its only purpose is for Sauron to dominate it using the One Ring. Yet she still has it and she uses it. The only possible explanation is that the elves are too strong to being dominated by these rings, and that they can use them freely as they wish, without any danger of being dominated by Sauron.
If this is so, then Sauron's plan failed even more catastrophically as he didn't foresee that elves could not be dominated this way. The movies give the impression of Sauron being an extremely powerful being, but it seems that he was not powerful enough to know that elves could not be dominated this way, and basically gave them three powerful magical items for free. Why wouldn't he know this?
(One could argue that he did know this, but he gave them the rings anyways in order to lure the other species to join in, that it was all part of the master plan, and an acceptable sacrifice. Whatever.)
So far the problems have been only minor. However, the One Ring just doesn't make any sense at all.
Sauron, an immortal almost-all-powerful being, puts most of his life force into a small magical ring in order to control the other rings. By doing this he basically makes himself extremely vulnerable and mortal. As seen in the beginning of the first movie, it's enough to cut his ring finger to kill him almost completely.
This makes absolutely no sense. Why would such a powerful being make himself so vulnerable and mortal, no matter what kind of world domination plans he had? Surely such a powerful being did know how vulnerable he would become if he makes such a ring? Wasn't it utterly stupid to do so, no matter how ingenuous the world-domination plan (which then even failed monumentally)?
Also, Sauron is able to sense the location of the One Ring when someone wears it, but not otherwise. Why wouldn't he make the ring so that he could always detect its presense?
And why was it enough to simply remove the ring from Sauron's body in order to kill him? How does that make any sense? Not only did the ring make Sauron very vulnerable, killing him was laughably easy: Just remove the ring and you have killed Sauron, one of the most powerful beings in this world.
Not only that, but destroying the ring itself would completely annihilate Sauron. Surely Sauron did know this as well. Why make himself vulnerable and mortal, and completely killable by simply destroying the ring? Even if the ring could only be destroyed in Mount Doom, why would he risk even that?
And related to that: Given that Sauron knows that the ring can be destroyed in Mount Doom, why was the entrance completely unguarded? Sure, the risk of anyone walking through Mordor with the ring and entering Mount Doom to destroy it was next to none, but why take even that little risk?
Why not seal the entrance completely so that nobody can enter? Or if the entrance must be kept accessible for whatever reason, surely he could spare a couple of dozens of orcs to guard the entrance? Or at least put a heavy locked door or whatever. With all the magic and wizards at his disposal, maybe some kind of magical barrier? Anything but leaving it completely unguarded and freely accessible, which is just plain stupid and doesn't make the least amount of sense.
All this seems to imply that Sauron was completely stupid, which doesn't make too much sense for such a powerful immortal being. Seemingly all of his minions (including Saruman) were also completely stupid because nobody seems to have suggested Sauron any of this either. Surely at least Saruman would have suggested to Sauron to seal or put guards at the entrance of Mount Doom?
The only power of the One Ring shown in the movies is making the wearer invisible. However, the movies hint at the ring having many other powers as well, but they are never specified. Some dialogue related to this doesn't make too much sense. For example Boromir says:
It is a gift. A gift to the foes of Mordor. Why not use this ring? Long has my father, the Steward of Gondor, kept the forces of Mordor at bay. By the blood of our people are your lands kept safe. Give Gondor the weapon of the enemy. Let us use it against him.
Exactly how was Boromir suggesting to use the ring against the forces of Mordor? Putting the ring on would only alert its presense to Sauron, and the invisibility does not help against the Ring Wraiths, so the ring would be basically useless for fighting against Mordor this way.
Ok, maybe Boromir did not know this. However, nobody else present at the time brought up the issue. (For example Gandalf did know this perfectly, as established earlier in the movie.)
Even with invisibility there's just so much you can do against an army of hundreds of thousands of orcs and other creatures. The invisibility of one person just doesn't sound like such a big power against an entire army. However, Boromir acts like he knows exactly how powerful the ring is. He (as well as others in other situations) makes it sound like it was some kind of superweapon which could be used to annihilate entire armies.
How exactly did Boromir know of the real powers of the ring, and why aren't they specified in the movies? Ok, this isn't a very big deal storywise, but it's a bit annoying because the viewer is kept in the dark and never told exactly how powerful the ring really is.
The prologue establishes that the existence of the One Ring was forgotten as millenia passed. Also, most importantly, the beginning of the first movie clearly establishes that Gandalf doesn't know too much about the One Ring. Even if he knows about its existence, he doesn't know about the details. He travels to Minas Tirith and searches old chronicles until he finds the story of the ring. All this is clearly new information to him.
However, later in the movie suddenly everyone seems to know about the ring. In a scene in the third film (only in the extended version), in a flashback which happens before the Council of Elrond (which happened in the first movie) Denethor, the Steward of Gondor, talks to Boromir (his son) and mentions about rumors of the One Ring having been found.
So the One Ring started by being a completely forgotten artifact which even the mighty Gandalf didn't know too much about, and even when he did find about it, he made sure its existence would be kept secret. Later it seems that everyone knows about the ring, and rumors about it having been found are widespread. Everyone talks about "the ring of power" as if it was common knowledge what it is.
While it might be remotely explainable how the knowledge got widespread after Elrond summoned the council at Rivendell (although the movie hints that at no point the reason for the council was specified to anyone), it still feels very inconsistent.
The Eagles don't make too much sense storywise. They help Gandalf out of the tower of Orthanc when Gandalf asks for their help, they help in the final battle at the gates of Mordor, they help taking Frodo and Sam out of Mordor after Mount Doom erupts.
However, why couldn't they help in other situations as well? Would it have been such a huge burden to carry the ring bearer and his party members to Mordor? Or if they didn't want to fly to Mordor, at least closer to it, so that they could have saved miles and miles of walking in dangerous enemy-infested territory?
In the first movie Gandalf definitely wouldn't want to go through the mines of Moria, so he wants to go over the mountains. That plan doesn't work, so they are forced to go through the mines. Gandalf keeps oddly quiet about the idea of asking the Eagles to transport them over the mountains. What would have been the problem?
I think the entire story would have been a bit more logical without the Eagles. They are too powerful benevolent flying beings, and thus they introduce too many unanswered questions about why they sometimes help while most of the time they are not even mentioned and nowhere to be seen, even though they could be of great help.
Gandalf's magical powers seem very erratic at best. And they also seem to depend on who he is fighting against.
When fighting against a mighty Balrog, an extremely powerful demon from ancient times, he can create a forcefield around himself which protects him from the Balrog's attacks, he can summon lightning, and overall he can fight as an equal to the mighty demon beast.
When fighting against the Ring Wraith, he can spawn a ray of energy from his staff which draws them away. When fighting against another equally powerful wizard (Saruman) he can use powers akin to telekinesis.
However, none of these powers are used or present when he is fighting against orcs. He just uses his staff as a bludgeon, but that's about it. No protective forcefields, no telekinesis, no summoning lightning... nothing.
It seems that Gandalf's powers fluctuate so that they are always in par with the powers of his foe. If his foe has no magical powers, neither does he.
(To be continued...)