In book two we are introduced to the allomantic alloy Duralumin which is obtained through an alloy of Alluminum 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.
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.
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.
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. Preservation probably deliberately set that whole thing up. Which brings up the interesting question of what happens if you burn atium around the crystals.
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).
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 the fur of an animal takes 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.
"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."
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 is also why a sufficiently filled metalmind is undetectable to Ironsight. Hemalurgic spikes rip out pieces of said Spiritweb.
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?
Also a Shout-Out to the arc number of 10. One Lord Ruler+Nine Mistborn= 10 original Allomancers