So, I've been rereading Lord of the Rings for the first time in a WHILE...

[Lord Joeltion]

I have an interesting story with LotR. I dare say, my experience with Tolkiens movies is kind of unique even.

I watched the movies before reading the three six what? yes, "six" books (plus apendixes) that make the proper Lord of the Rings saga. Buuuuut I already had read not only the Silmarillion AND The Hobbit, but also some other apendices on the worldbuilding. Basically, I was in the middle of reading the complete saga and skipped LotR because reasons

Spoiler: reasons? Yeah... "reasons"

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I knew I wouldn't finish LotR by the time the first movie came out, so I wanted to avoid confusion about "source" material. Also, I actually prefer Silmarillion and the Hobbit.

Anyway... I read the books "after" watching the movies (altho I already knew most of the backstory not shown in the film). And when I read it... I finally understood why the movie adaptation changed most of the things it changed.

Tom Bombadil is obvious: his plotline is basically transversal to the main story. He appears in the book as pure worlbuilding, which is totally fine in book format but a no-no in movies.

The changes on motivations and backstories of hobitses (?) is probably related to the extense time period the book deals with (25-50 years in the Shire; about 3-6 months until Rivendell, etc) and the relatively shrinkage of time in movies (most movies skip minutes-years from one scene to another without any caption or hint whatsoever). It's understandable they look more bland in the film: most of the character bulding is detailed in the books pretty much in several micro-scenes and memories/jokes/talks between them... which for obvious reasons can't translate at all in movie format.

The same constraint applies to Saruman and Elrond, but for different reasons: They aren't as much relevant characters as the fandom usually think. Yeah, they are great characters, deep and complex... but their intervention to the main story is pretty much scarce in comparison. In relation to the quest of the Ring, they serve as obstacles and a helping hand to the main characters. For cinematic purposes alone, you don't really need to know Saruman's motivations and complexity because he is supposed to be The Dragon, in the story. You simply need to know how powerful and mean he is. Just like Darth Vader. Elrond's (and elves in general) grudge towards Men are tangential to the plot, and also dissonant to the depiction the movie choses for Elven People IMO (in a movie, you either show them as Demi-Angels or Demi-Pricks; you can't both). So I can live with an abridged version of them on the big screen. I hurts the fans more than it really hurts the movie.

Now I agree with OP that Vigo Mortensen was a "miscast". But I also found a justification comparing the movie to the books. Basically, I think Vigo fits better with the Aragorn they tried to depict in the movie; more than the one from the books. I always thought the book!Aragorn was an older one (not agewise, but in experience/mind). He pops in the story basically knowing what his destiny is and what he was. He holds no insecurity, no fear, no shame. He is proud and constantly shows to Men what the ancient rulers looked like. Even in dirty rags, he stands out among kings and lords. Now, that is everything the Classic Hero was at the time the book was written. And while it's totally perfect for the story.... sadly the movies were released in the 2000s. And Aragorn is supposed to be "Da Hero", if not the protagonist (that would be Frodo). He is the ****ing middle of the posters. So, I think... he was reworked to make him more appealing to modern audiences: a "broken" hero, not so full of himself, and basically, much more relatable to us. book!Aragorn was supposed to appeal the Post-War Common Guy/Girl. Which is fine. I can still relate to that too. But it's not how marketing works in Hollywood. Was it a bad choice? I don't think so. It's not like he was totally deformed. It's just a kind of younger Aragorn, "cooler" if you want, but still pretty close to what book!Aragorn might have been in at some point.

But it's totally my personal experience with the movies/books; so you may take my opinion with a grain of salt if you want.