<p>galoisien has raised a set of interesting points in post #8. To that, I'd add other comments about the thematic content of the work:<br>
Foremost, there is the central role of mercy (a point discussed explicitly by Tolkien);
but then also mortality and immortality;
redemption, which occurs multiple times;
the rejection of simplistic dualism, while keeping good vs. evil as the central conflict;
the role of "fore-knowledge" in causing despair, and the reactions of characters of different mettle and type to despair (Boromir, Denethor, Saruman--and Erestor, for that matter);
hope;
the turning away from power that could be taken;
and of course, the role of the small and weak in confounding the powerful, as well as courage, loyalty, friendship, and love.</p>
<p>The character development also has depth. Of course, there is the coming-of-age archetype and the Quest, and hence (part of) Frodo's development. Frodo goes beyond the archetype, though, I think. The relationship between Frodo and Sam has rather intense British socioeconomic class-related overtones, which have occasioned a lot of debate. </p>
<p>Tolkien's work is unusually extensive as an act of "sub-creation." The Silmarillion provides back-story already conceived but not published when LOTR appeared. Thus, the story of Aragorn and Arwen alludes to the story of Beren and Luthien. The Silmarillion and Unfinished Tales provide back story on Galadriel's exile.</p>
<p>As far as linguistic depth goes, the journal of the Elvish Linguistic Fellowship (Vinyar Tengwar) is indexed by the Modern Language Association (MLA). Tolkien drew on his command of linguistics in creating the various languages, as well as the changes in the names of some of the characters over time, and the variations in the names of characters as they were known to different groups.</p>
<p>The comment has been made that LOTR is a "profoundly Christian, and even profoundly Catholic work." I will not touch on that, except to observe that I think it is no coincidence that Frodo comes of age at 33.</p>
<p>(My apologies to the true LOTR afficionados: My comments in this post are inevitably somewhat of a hack job, but they might serve to indicate some of the depth that is in the work itself.)</p>