Marasi is the anti-Vin. Vin was a tomboyish Action Girl who nonetheless had some traditionally girly interests, like dancing and wearing fancy dresses and perfumes; Marasi is a girly-girl who nonetheless has some traditionally masculine interests, like shooting and criminology. Vin was jaded and streetwise but had little patience for formal scholarship; Marasi is sheltered but very well-educated. Vin was a Mistborn, the most powerful form of Allomancer but is frequently forced to work around the limits of her powers; Marasi is a Pulser, considered one of the weakest forms of Allomancer but manages to be key in defeating an Implacable Man with clever use of her abilities. Confirmed by Sanderson, partially.
Ranette's hazekiller rounds. The anti-Coinshot rounds are the same trick Vin used on a pair of Inquisitors near the end of Mistborn: the Final Empire: something sharp attached to a bit of metal so that it will slip off the back when Pushed without affecting the weapon's trajectory. Vin used broken arrows and metal rings, Ranette used metal-jacketed ceramic rounds.
The reason why enough Terris blood ended up in the general population within a few hundred years to make Twinborn and Ferrings? The Luthandel Ska and Nobel refugees were living in close proximity to the Terris survivors during the end of The Hero of Ages. Similarly any Terris people near any of the other survival caches/shelters would end up mixed in with the Nobles and Ska there. In the chaos and desperation of what they though was probably the end of the world, it would hardly be a surprise if a whole bunch of people with mixed heritage were born some months later. Even after the Harmony reforms the world, all of these people would contiune living in close proximity in the Elendel Basin, resulting in the mixing of Allomantic and Feruchemist bloodlines.
An addendum to the above: why are no "true" Feruchemists or Mistborn (just ferrings, Mistings, and Twinborn) encountered? All of the Mistborn and Feruchemists in the world by the end of The Hero of Ages had been killed by Ruin in one manner or another, save Sazed. On top of this, almost all of the nobility in Hero of Ages had been killed, either in the chaos or by Ruin's Inquisitors to get spikes, so the Allomantic bloodlines were diluted even more. The only istborn left by the end of the original trilogy is Spook, and he wasn't actually born a Mistborn. It's a plot point that all of the kidnapped women can trace their lineage back to them. The original trilogy mentions at a couple points that Allomantic powers have declined significantly since the beginning of the Final Empire. In the beginning, any emotional Allomancer could control koloss or kandra (as Elend is able to), but by the time of the books, it could only be done with the help of duralamin. Plus it's noted that the Lord Rulers attempts to weed out Ferruchemy had largely been quite successful.
People wear earrings while praying to Harmony. When Wax prays to Harmony, he feels like he is detecting a faint response (and eventually gets some more direct responses). This is because the earrings are Hemalurgic spikes, just with little or no Hemalurgic charge note Word of God is that the earrings are melted-down-and-reforged Inquisitor spikes, and leaving them out most of the time let all but the bare minimum Hemalurgic charge drain out. Harmony has Ruin's power and thus his ability to communicate with those pierced by spikes (plus Preservations power to actually hear when they ask him something). The Path dictates that the earring should be worn during daily prayers (which are really just reflection) or while undertaking actions of great import, so that Harmony can nudge people when he needs to, particularly at critical moments.
Regarding the tech level and the path - it seemed a little weird at first that they'd get to electric lighting and motor vehicles so quickly after the Dark Ages that were the Final Empire. However, at the end of Hero of Ages, Sazed didn't just leave them with the knowledge of the Keepers. He gathered all of the survivors into one place ideal for founding a new settlement (take a look at the early chapters, the narration directly comments on how incredibly fertile this area is, for example - and clearly it's in fairly easy reach of quite a lot of resources), and a whole new religion was founded based around him. Given that it's based around him, we can assume the philosophy of it stems from that of the Keepers - that is, the followers would place a powerful emphasis on the discovery and cultivation of knowledge. That pretty well creates the atmosphere to have a very, very long period of time that is basically an combination of an industrial revolution, the Renaissance, and the Enlightenment. Even 3 centuries later, the one adherent to the Path that we do see is an extremely gifted investigator. Also keep in mind that the tech base of the Final Empire was actually quite advanced; it's just that some specific knowledge was suppressed to allow the Lord Ruler to remain in control. They had functioning pocket watches, indicated very precise understanding of machinery, and had canneries, indicating solid knowledge of basic industrialization. The engineering projects that the Lord Ruler ordered in his storage caches indicate advanced understanding of construction and architecture. Overall, the tech base was more akin to late 1700's/mid 1800's, but with some significant gaps in areas like chemistry and gunpowder technology. Therefore, it is entirely plausible that in three centuries, the tech base would reach early 1900's despite the need to rebuild all of civilization essentially from the ground up.
A meta-one: most angelic names (Raphael, Gabriel, etc) end in -el, meaning 'of God'. Look at which place names in Alloy of Law end in that suffix: Vin's and Elend's. Vin, because she WAS god for a bit and she was chosen by BOTH Ruin and Preservation, in their own unique ways, and Elend, because HE was Emperor, which due to the Lord Ruler was traditionally a God-Emperor position, and because he's also the chosen of a god. Vin.
Interestingly, this is purely for the reader's benefit, as Scadriel has no moon.
A bit of symbolism with the power choices for Wax, Wayne, and Marassi. Wax and Wayne both have powers that make them, personally, incredibly dangerous (albeit in differentways). They're individualists from the roughs, and used to doing the whole lone hero routine. Marassi has a power that is hard to use alone but amazing if you work with one or more partners. She's from a city, and focused on law and community.
In Well of Ascension, Elend built his new government as a representative republic so that the people would have a voice, with a king to add a sense of stability. While he made some mistakes (most notably not granting the king any emergency powers), the concept itself was sound. Harmony took it to its logical conclusion: The people have a democratic republic, but there is a god above them providing stability. He makes a few minor adjustments here and there, but mostly doesn't interfere unless there is an existential threat to civilization. When such a threat does arise, he sends in agents to fix things.
Steris contract with Wax can seem odd going as far as to include mistresses and specific lengths of time they can meet. We later learn her father had a child with a mistress, this isn't the first time she has had an arranged marriage and she annoyed her other finances to the point where they left her with letters sent of all her faults. She presumed Wax would get annoyed by her as well so she only left small amount of times to interact. She also presumed Wax would cheat hence the mistress section.
The Ars Arcanum, presented as an in-universe document, notes that Hemulurgy is a Game-Breaker. Hemulurgy is Ruin's magic system. Ruin essentially broke magic posthumously.
While Odium is often speculated as the "Foreign God," Trell, others have speculated that another Shard, Autonomy, has been the one responsible. They have theorized that this falls in line with Bleeder's distaste of Harmony's control in Shadows of Self. This also requires the knowledge that the infamous red eyes of Voidbringers and the mysterious beings at the end of Bands of Mourning aren't necessarily of Odium, just of corrupted Investiture.
Alternative Title(s):The Alloy Of Law, The Mistborn Adventures