Alternative Character Interpretation: Are the Pie family holidays simply born out of tradition, or is it an indication of poverty? The Pies only use things found on their farm for the holidays, while Applejack uses decorations which presumably had to be bought. Also note the looks on the Pies' faces as Applejack tries hard to drink her rock soup. This would also put another dimension to the Pie family's outrage at Applejack's decorations, since that could be interpreted as being shown up.
Broken Base: While Marble Pie is Ensemble Dark Horse, some fans find the fandom's obsession with shipping her and Big Mac relatively frustrating, especially since some feel she and Big Mac get shipped mainly because of her being a similar Expy to Fluttershy.
Ensemble Dark Horse: Marble Pie, who, out of one of Pinkie's three siblings, has the smallest role in the episode and no lines at all, quickly became popular. The Moe factor and the ship with Big Mac help.
The Pie family didn't have too much characterization before this episode, especially Limestone and Marble Pie, who tended to be used as OC Stand Ins or made Expies of Maud. Now that they got some in this episode, expect their canon personalities to show up much more.
The existence of an Equestrian counterpart to the Quakers is an opportunity for a lot of Worldbuilding. For example, how and why they broke off from the mainstream pony society.
Pinkie's calculation to get to 19 new friendships is mathematically sound: there are 4 * 6 = 24 ways to pair up the Pies and the Apples, and she properly discounts her friendship with the Apples, and AJ's friendship with Maud.
Maud mentions that corundum is higher on the Mohs Hardness Scale (itself not always common knowledge) than quartz. Corundum (hardness 9) is crystalline aluminum oxide and the primary component of rubies and sapphires. Unlike quartz (hardness 7), it is hard enough to cut glass.
And she says they're looking for a rock that looks like rapidly cooled lava, ie obsidian, a shiny black rock just like the picture.
She notices bits of rock on Applejack's hoof and immediately identifies it as andesite, a mountain rock. It takes its name from the Andes mountain range in South America.
Limestone is a sedimentary stone that, when subjected to great heat and pressure (usually during mountain-building events) recrystallizes as smoother-texture, larger-grained marble. So, one could say Limestone is a rougher version of Marble.
Harsher in Hindsight: Marble Pie having Ship Tease with Big Mac at the end of the episode becomes a lot harder to watch after watching My Little Pony: Best Gift Ever where Marble is heartbroken seeing Big Mac and Sugar Belle kiss under the mistletoe.
I Knew It!: Ever since Marble Pie was first seen as a filly with her mane hanging over half of her face, the fandom frequently portrayed her as a Shrinking Violet similar to Fluttershy. This episode confirms that characterization.
Informed Wrongness: Applejack taking it upon herself to make the two families celebrate Hearth's Warming the Apple way. The Pies don't seem very happy with their own traditions, and we know from how Pinkie got her cutie mark that with the right gesture they can come around and really enjoy themselves. The Apples' ideas of how to celebrate (a big fancy meal, giving the youngest child a special privilege, exchanging presents without having to earn them) also line up better with how most viewers will think of Christmas and sound more appealing than the Pie traditions at face-value, so it's not hard to understand why Applejack would think that they would appreciate it once she showed them her way of doing things.
A disadvantage of this episode is the way Pinkie makes the same mistake she did when introducing Maud to her friends — she hypes up the things her family has in common with her friends without considering that their differences could become a source of conflict. Though the real source of conflicts in both episodes didn't come from what they had in common—and in this episode, things the families had in common (like Apple Bloom's and Maud's "a lot in common about dreaming about turning into things") were what started to bring the family together before Applejack, like Pinkie in "Maud Pie", tried something dramatic.
Like "Party Pooped" (which was also written by Nick Confalone), this episode derives a lot of Cringe Comedy from characters becoming offended by disruptions to their traditions.
Moe: Marble Pie is an even bigger Shrinking Violet than Fluttershy. She doesn't even get any dialogue except a few "mmhmm"s. It makes one want to be like Pinkie Pie and hug her.
Older Than They Think: The official names of the Pies were given in Pinkie Pie and the Rockin' Ponypalooza Party and The Elements of Harmony guidebook, both published two years before this episode aired. G.M. Berrow clarified before this episode aired that she did give the names to Limestone and Marble (Maud was mentioned but not named), so that Marble, Limestone, and Pinkie would have initials "MLP" (read: My Little Pony). This episode canonizes those names.
Unintentionally Sympathetic: Applejack. The episode is supposed to be about her learning to respect the traditions of others and not be more open-minded about the Pie family and their own traditions. However, this glosses over that none of the Pies, not even Pinkie, ever bother to explain their traditions to the Apples and don't show any interest in what traditions the Apples might know. Applejack was trying to make the effort to get along with the Pies, but they did little to make her feel welcome.
Viewer Gender Confusion: Limestone can easily be mistaken for a boy, with her very gravely sounding voice, gruff behavior, and non-indicative name, as long as you don't remember Pinkie previously talking about only having sisters.
Applejack's attitude towards the Pie traditions, their consequences, and the conclusion she ultimately reaches sound a lot like a discussion of forcing assimilation on immigrants and cultural imperialism.
Some fans have interpreted this episode as being about a class conflict between an impoverished family and a "privileged" one. Of course, since the Pies are prosperous enough to keep living on the Rock Farm for generations instead just moving out and to attribute their continued good luck to Holder's Boulder, that doesn't seem to be the case. Especially since Trixie earned a lot of money after working there for a short period.
: there are 4 * 6 = 24 ways to pair up the Pies and the Apples, and she properly discounts her friendship with the Apples, and AJ's friendship with Maud.