It seems odd that guards in a pseudo-medieval/renaissance-tech setting only wear breastplates instead of full mail, helms, greaves, and so on. Even with the obvious threat of Allomancers and Mistborn, if you're going to armor up against mundane threats, you'll want full protection. But if you're just wearing a breastplate and an Allomancer shows up, all you need to do is cut the straps and you're no longer immediately helpless against an Allomancer, while a fully-armored soldier will never get all his gear off before an Allomancer uses him for all manner of hilarity. Going around with just a breastplate is an effective compromise between being armed for mundane threats and still being of some utility against an Allomancer. In fact, breastplates are apparently designed with emergency release straps for just that reason.
There's also the fact that full plate/chainmail armor is expensive, and not usually something you'd outfit non-elite soldiers with. While the upper nobility could afford full plate armor, it probably wouldn't be worth the investment considering how easily most combat Allomancers could kill a man in heavy armor. It makes sense that most armor would be kept to economical munitions breastplates and helmets.
"Elend" is the German word for "misery" or "hardship." Also, the philosopher Kierkegaard said: "To venture causes anxiety, but not to venture is to lose one's self."
The Lord Ruler structured the Final Empire to be as stable and rebellion-resistant as possible. Fantasy Gun Control is in full effect because the Lord Ruler wanted to make it difficult for anyone but professional soldiers (of whom he would always control the majority) to be deadly at range. That leaves Coinshots as the only option for ranged attackers among the Skaa. To shore up that weakness, he made it illegal for nobles to have children with Skaa. But he also covered the other side of the equation: Skaa are paid in either direct resources (like food or clothing) or in wooden chits, meaning that even Skaa Coinshots who evade the Canton of Inquisition don't have access to coins. The only exception? The Lord Ruler's soldiers.
Everyone in Kelsier's crew was given a nickname, except for Vin. But he did give her a new name: Valette. Going further, there's only one person in the crew who doesn't use their nickname: his brother Marsh. It's a subtle way of showing that she's family.
Copperminds store memories, and Feruchemists pass down knowledge by memorizing the lecture of someone tapping another. Copper => Copy.
Allomantic pewter is made with lead. It's not poisonous to an Allomancer who burns it quickly, but it's the metal most likely to directly harm the user, with pewter dragging.
Why did Ruin give Marsh of all Inquisitors the most spikes? Besides the painful irony of fighting a former friend, he helped Vin kill the Lord Ruler, one of Ruin's obstacles.
A bit of Fridge Horror: before Vin joined the crew, she replenished her Luck through trace metals, like pewter mugs. Allomantically burnable pewter is a mixture of tin and lead; there's got to be a lot of lead poisoning going on.
Another piece of Fridge Horror: Mistborn and Lurchers would suffer from anemia if they burns away all their iron. Same goes for other essential minerals.
Throughout the series, there's a theme of impulse and caution both being needed to achieve goals; both achieve great things, and neither is shown to be better than the other. It's a subtle nod to how both Ruin and Preservation are needed to create life, and how the Hero needs the power of both.
Brash action gets Yeden and most of the skaa rebellion pointlessly killed, but it also inspires the whole of Luthadel to revolt in a battle that they could win. Conversely, excessive caution made the rebellion a minor annoyance, but a patient infiltration undermined the whole system.
Look at how Vin and Marsh killed the Lord Ruler and the Inquisitors. Marsh discovered the Inquisitor's weakness by remaining undercover and waiting for a chance, killing all but one in their sleep, but when he tried to remove the Lord Ruler's spike, he was blindsided by the fact that TLR didn't have one. Vin charged into Kredik Shaw with half of a plan and got captured early on, but once Marsh freed her, she breathed the Mists and killed the Lord Ruler.
Early on in Final Empire, Kelsier notes that the skaa are terrified of the mists, and thinks "I'll have to do something about that." Near the end of the book, he's gained the title "Lord of the Mists," and while skaa are still scared of them, they are no longer so frightened that they refuse to even leave their houses at night, since they see the mists as Kelsier's protection.
At first, it looks like Allomancy having a hard time Pushing or Pulling any metals partially or completely inside someone's body is a matter of plot convenience. It would be too easy if Mistborn could rip the metals out of someone's stomach or the spikes in a Inquisitor's head. There's an unstated reason for this, though: the Spiritweb is made of Investiture, which is also what Allomancy is driven by, and Investiture resists other Investiture. This also explains why powerful allomancers like the Lord Ruler are still able to push metals inside someone. It was never impossible, it just required more power than modern allomancers had.
In book two we are introduced to the allomantic alloy Duralumin which is obtained through an alloy of Aluminium and 4% copper, and it is mentioned that the aluminum comes from a recovered noble's silverware. This may seem to be an odd metal to make silverware out of, until you realize that aluminium can't be pushed or pulled, so not only is it super expensive (making it a display of wealth), but it's also less likely to be used by a coinshot assassin to kill you from a distance. Aluminium also used to be a rare metal before the discovery of the electrical process to extract it from bauxite. Aluminium used to be more valuable than gold and it was used as expensive cutlery in real life.
There's some brilliant foreshadowing that OreSeur is TenSoon:
When Vin is researching Alendi's logbook, OreSeur is confused until she explains, and Vin asks if he remembers when she explained all the details around the logbook and Alendi's fate. OreSeur then says that he remembers hearing her mention it briefly, and Vin passes it off, as Renoux was not truly part of the crew's planning sessions. At first glance, this seems like a simple bit of exposition to remind the reader of these characters' relationship in the first book. But TenSoon was never there for the revelations from Alendi's logbook, so he really wouldn't know. The passing line by him is really a very quick, very smooth act of evading a stumbling block that would reveal who he really was, and only noticeable in hindsight. When TenSoon said he was good at hiding his true nature, he wasn't kidding.
Much earlier as well in the novel, when TenSoon is first introduced as the wolfhound, he apologizes to Vin, saying that he forgot to mention that putting on fur would take a while. If you were following closely, OreSeur DID tell Vin that impersonating an animal would take several hours longer than a human because of the fur, but TenSoon wouldn't have been able to know that. He assumed that Vin thought it took longer than normal because he manipulated a voicebox into the animal body.
There's also the shift in Vin and OreSeur's relationship, as she begins to open up to him, ask him to open up to her, and treat him better. Part of that is Vin realizing how she's been mistreating (one might even say "prejudiced against") her kandra, but what really makes her start shifting gears is an offhand comment from OreSeur. A comment the real OreSeur probably would never have made, because he'd grown to resent Vin too much. Also, as shown in the Funny page, TenSoon has a much better sense of humor than OreSeur.
It's noted that Zane self-harms because the voice in his head backs off somewhat if he does. It's easy to dismiss this as a symptom of his insanity, but Ruin is actually influencing him throughout, and as we see later in the book, Marsh is also able to successfully resist Ruin during acts of Violence. So when Zane commits acts of violence (even on himself) he is able to, at least temporarily, ignore Ruin's influence after acts of violence.
In Hero of Ages, Vin, not knowing the name of the malignant entity she inadvertently freed, starts calling him "Ruin" because it just seems to fit. This is, of course, said entity's actual name. Was it just Vin making a lucky guess? Nope; we find out later that Ruin actually rather likes Vin because of how deadly she is, and he can speak into her mind via her earring and be subtle enough she can't tell it from her own thoughts (or her memories of Reen). She knows his name because he told her.
Why Allomancy shatters the crystals in the Pits of Hathsin: Allomancy is the power of Preservation, and the crystals in the Pits gather the body of Ruin, so the powers are diametrically opposed. Allomancy would naturally disrupt Ruin's power.
The method of harvesting atium requires descending into a dark crevice and reaching into deep holes filled with sharpened crystals. The whole process is horrible, brutal, and shreds the arms, typically killing the prisoners assigned to the work. But of course that's how it works; atium is the body of Ruin, so of course harvesting it would be destructive.
Feruchemy. This magic system wasn't directly created by Preservation or Ruin and instead came about seemingly naturally in humans. But remember, humans, while of both Ruin and Preservation, are weighted in Preservations favor, making it rather obvious why the ability that manifests in them naturally is one which allows people to preserve traits they have to call upon at a later date. But the fridge doubles when you consider that this ability actually changes them and their capabilities physically, as is most obvious when a feruchemist taps strength. So not only does the magic preserve their traits, but it then uses those traits to change the user down the line, while allomancy doesn't physically change the user. And as has been stated several times, ruin is necessary to counter preservation, thus allowing things to change.
The Lord Ruler and Feruchemists:
Feruchemists were ruthlessly hunted for decades or even centuries, because the Lord Ruler feared what would happen if A Mistborn/Feruchemist was born. So why did he change course, let the Terris survive as eunuch stewards, allow them to wear the metal needed for Feruchemy, and risk allowing a remnant of Feruchemists to survive? So he could harvest their powers for his Steel Inquisitors. We know that they were granted at least Feruchemical gold. Essentially, he turned the Terris into breeding stock to augment his servants.
The Lord Ruler prohibited the Terris people from being allowed to touch metal because Allomancy and Feruchemy use the same set of metals. He probably waited until he was sure the other metals were forgotten before setting up the stewardships. If the Keepers had known he was trying to keep things like duralumin secret, they probably would have arranged to disseminate the knowledge, say by anonymously spiking nobles' drinks (the same tactic the Steel Ministry used to look for atium Mistings). And then Ruin would have been able to harvest those Mistings for their Allomantic enhancement and temporal powers, instead of wasting a Mistborn for the same (since the best use for a Mistborn, Hemalurgically speaking, is for the ability to burn atium).
When Vin is attacking Cett's keep in The Well of Ascension, she takes startling pleasure in assaulting and killing Cett's troops and exercising her Mistborn abilities. Afterward, she has a My God, What Have I Done? moment where she's horrified by how she killed all those men. While she's doing so, she's holding her earring in her hand instead of wearing it. The earring is a Hemalurgic spike, and Vin's satisfaction at such destruction was Ruin's influence over her. When she took the ring out, Preservation's influence over her gave her that sense of horror at what she'd done.
A subtle bit of brilliance about Sazed being the Hero of Ages and becoming Harmony. Feruchemy is a balance of Preservation and Ruin, and Sazed, a feruchemist, takes up both Preservation and Ruin to become Harmony.
In Hero of Ages, it's briefly mentioned that there were nine original mistborn. The Lord Ruler used lerasium to give powerful kings and nobles alomancy in order to bribe them onto his side. Say, wasn't there anotherEvil Overlord who offered nine mortal kings great power in order to manipulate them into serving him? Coincidence, or Shout-Out?
He also turned the world into a smoke and ash choked near wasteland and he was defeated by having the jewelry that contained his lifeforce stolen, albeit not thrown into a volcano.
Also a Shout-Out to the arc number of 10. One Lord Ruler+Nine Mistborn= 10 original Allomancers.