All the valar come in pairs, with three exceptions: Ulmo, who chose to live alone, Nienna, and Melkor. It is stated that Nienna is sorrowful and patient, and weeps for the wounds of the world. Could it possibly be that Melkor is her lost love? That would make her sorrow even more poignant. Her patience seems infinite, also when we remember that Melkor actually wanted Varda for a spouse, but she rejected him. So Nienna can`t do anything but weep.
When Thingol heard about the massacre of his kin (the Teleri) by the Noldor, he banned the speaking of their language (Quenya) throughout Beleriand. This meant that the Noldor (whom he no longer trusted) had to speak in a language he and his people could understand, or risk being shunned. It also meant that, while he could not demand that the Noldor get rid of the swords that had killed his kin (since they were needed in the war against Morgoth), he could still punish them by banning the language in which the orders to kill the Teleri were given.
Meta-example: The Silmarillion itself was compiled from Tolkien's notes, many of which were not entirely consistent with one another... just like different versions of real-world mythology.
The elves are described dwelling far beyond the sea, on the other side of impassable mountains, beneath the earth in elaborate underground palaces, and in the deep, enchanted forests. In other words, in exactly the places where the Celts used to believe The Fair Folk to dwell. This is most definitely not a coincidence, but a deliberate attempt to tie The Silmarillion to the real world mythology.
The three elvish ambassadors to Valinor seems to have a root in mythology and folklore.
Ingwë, the leader of the Vanyar, is an equivalent of Ingwaz or Frøy. In norse mythology the elves live with him in Alfheimr. The vanyar live with Ingwë in Elvenhome, and hey is there a nominal resemblance between the Vanyar and the Vanir (the people of Frøy).
Finwë was called Finn in the early drafts. Wayland the smith is the son of the Finn king in the norse poem, and is called "Visi alfa" (elven lord). The noldor are master smiths, as wayland was.
Elwë has a name that rings almost like "Elf". His people, the Sindar, seems to be the most traditionally "elvish" elves in the mythology. And like the woodland elves, they prefer twilight and dusk, being called the Grey Elves because of this.
Not sure if this was intentional on Tolkien's part or not, but while Morgoth is Big Bad for the overall story, each of the three "Great Tales" which form the most significant arcs of the mythology features a significant role from one of his three chief lieutenants- Sauron in "Beren and Lúthien", Glaurung in "Children of Húrin", and Gothmog in "The Fall of Gondolin".
Which characters get the primary focus is something that changes throughout The Silmarillion- first the Valar, then the elves, then men, and finally at the end of "Of the Rings of Power", the hobbits. This reflects Middle-Earth's gradual transformation from a purely mythological world inhabited by embodied elemental forces into a a more mundane world like out own (or possibly our own altogether, if some of Tolkien's Literary Agent Hypothesis comments are to be taken seriously).
It is mentioned that Elrond kept and compiled the stories of the Elves in Middle-Earth in Rivendell, and that Bilbo translated works from Elvish to the Common Tongue while he was in Rivendell. Presumably, The Silmarillion was written by Elrond, translated into the Common Tongue by Bilbo, and then translated into English by Tolkien. That would explain why it is told from an Elvish point of view, though with a very generous regard for select humans.
This also leans into the fridge horror part, during Sauron's stay with the Númenóreans he tells them about the divine lord Melkor and all his wonders and a lot of them end up believing him. Why? Because Melkor hasn't gone by that name since the beginning of the first age, and hundreds of years have went by since then, where the somewhat younger Númenóreans might have only heard his tale through the name Morgoth.
It's more likely that Sauron called his master by his original name because of the meaning. Melkor, meaning He Who Arises In Might - awesome. Morgoth, meaning The Dark Enemy Of The Workd - not so much. It is as if some dark cultist would introduce Satan as "Lord of Lies" or something like that instead of "Lightbearer". Of course, some people fall for the more sinister name, but if you want to go for numbers, pick the one that sounds more appealing.
In The Silmarillion, the Dark Lord Morgoth imprisons Húrin and forces him to watch from afar his family be destroyed by Morgoth's curse. Among other misfortunes, Húrin's two surviving children unknowingly marry and conceive a child. Just how much did Morgoth show Húrin, exactly? It makes it even more amazing that Húrin never screamed once.
Possibly the biggest mind screw in all the book but when Eru told Melkor that all things he thought he could make were merely an extension of Eru himself I realized that the whole fall of Melkor and his subsequent evil agenda were actually anticipated and factored into Eru's plans for the creation of the universe.
Eru's flame of creation, the only thing capable of giving free will and true life would be pointless if living things didn't have evil to choose apart from good. Melkor's deepest desire, of having that flame, which eventually drove him to madness actually stems from his existence being complementary to it.