I would encourage anyone to watch it but since I found it interesting I'll summarize at least what I got out of the lecture.
The ideas below are not my own but is my summary of Michael D.C. Drout's lecture
A topic which is common across a lot of Tolkien discussions is how the book creates a sense of consistent reality that occurs within the universe. As if The Lord of the Rings was a real history, written, re-written, and amended through time. This sense of true history and the manner in which the story is told creates a sensation of learning and generates nostalgia for the past but of course the past is lost. How does Tolkien achieve this? Well by being a philologist.
Creating a sensation of history
Tolkien was building a textual ruin, practicing the art of "literary tribology" as Drout puts it. Culturally in many cases we are fascinated by aging of things, we find ruins beautiful, we distress jeans, people even pay to have their guitars "relic"d.
But how does one create a textual ruin? By copying the features of real textual ruins, like Beowulf. Real textual ruins have a sense that though incomplete the texts were once coherent, even 'when written by multiple hands over time. They contain cultural references which are consistent and which persist over time.
Drout distinguishes between a "Broken Refernce" and a "Pseudo Reference" the latter being a reference essentially made up which has no other source (like the cats of Queen Berúthiel at the time it was written) while the former is a reference with some external story which is told elsewhere (such as the references to Húrin and Túrin). All of the references in the Lord of the Rings (save the cats at the time of writing) are "broken references". These broken references are mostly recognizable to the reader as different, but especially so when they are used in a consistent fashion. At no point in Beowulf, Drout explains, do the broken references conflict internally or externally with other texts, despite being a story with trolls and dragons.
This use of broken references also creates a sense of "reverse dramatic irony" when the characters know more than the readers (that is, they understand the reference, no one goes "the cat's of Queen who?"). Many of these broken references also appear to be "poetic devices" being often verse or at least containing alliteration, meter, or rhyme. This is common in real "textual ruins" because alliteration, meter, and rhyme, etc., are of stable references because the structure makes them easy to remember they tend to persist when stories are re-told or re-written.
The re-telling and re-writing which occurs in textual relics is another device which Tolkien deploys. The Lord of the Rings has a frame narrative and is in fact a revised version of the Red Book, already containing several layers of textual history in itself -- let alone the references to other stories outside of the text. Tolkien employs different narrative forms, there's spontaneous verse and translation, high brow prose, but also hobbit jokes and songs. It's a book that seems to have many authors. In fact, there are even some textual inconsistences which are evidence of multiple hands. For example, who is oldest, Bombadil or Treebeard?
but alsoGandalf, The Council of Elrond, FotR wrote: But I had forgotten Bombadil, if indeed this is still the same that walked the woods and hills long ago, and even then was older than the old. That was not then his name. Iarwain Ben-adar we called him, oldest and fatherless
Gandalf, The White Rider, TTT wrote: Treebeard is Fangorn, the guardian of the forest; he is the oldest of the Ents, the oldest living thing that still walks beneath the Sun upon this Middle-earth
The Least Knowledgeable Character POV
Drout points out that Tolkien achieves this sensation of textual history without completely losing the reader and also without having the dreaded "Chapter two: on the people, history, culture of my fantasy world". He uses a technique which Drout calls the "Least Knowledgeable Character Point of View". Tolkien rarely uses an omniscient third person narrator and instead in almost all of the chapters chooses the Least Knowledgeable Character (LKC) as the reader's point of view.
By doing this, as the LKC goes through the scene anything which is important for them they must learn and consequently we learn as the reader. In that way what is happening to the character who's perspective we're reading, learning, is also happening to us, the reader. So in addition to this sense of learning it also gives Tolkien an opportunity to do exposition about the world in a manner befit of a textual ruin which surely would not contain such a "chapter two".
End of Summary
I found Drout's lecture to be very interesting (obviously I made a post and summary about it). When asked about what I enjoyed about Tolkien I've always pointed to the sense that it's "history" which is often not present in other fantasy, at least in the same way. The consistency of the references (e.g. that you don't explicitly contradict yourself) was easy enough for a non-philologist like me to pick up on. But it didn't seem a sufficient response for why The Lord of the Rings had this compelling truth feeling to it. I've read a handful of Tolkien analysis books (mostly Shippey, Flieger, Scull & Hammond) and it's quite possible that this material is present in those works but this was the first time I've seen such a excellent summary of how he creates a "textual ruin" and consequently why it's such a popular book among so many people. If you enjoyed any of my summary (even if you didn't) I highly recommend watching the lecture he does a much much better job than me.