Did Twilight really not realize that Rarity had twice tried to blow her off for the Canterlot elite (once with the dress, once at the party)? Or did she know all along, and feigned ignorance to help Rarity save face? Or was the boorish behavior of her and the rest revenge?
Were the mane six socially clueless at the Garden Party, or were they serving alcoholic drinks at Twilight's party and had gotten slightly inebriated? Maybe that's why Spike wasn't present…
Did Fancypants recognize Twilight Sparkle for who she was? He gives a mysterious wink when he refers to her as "this lovely young filly from Ponyville..." If this is true, he may even know a little bit about her personality — particularly that she dislikes special treatment and that she wouldn't appreciate him announcing her identity.
Is Fancypants helping further Rarity's social career from sheer good nature and altruism, or is he just enjoying the way she gets under the skin of obnoxious social-climbers like Jet Set and Upper Crust?
Crazy Awesome: If the "Party Cannon" doesn't make Pinkie Pie this, then nothing ever will.
Ensemble Darkhorse: This episode introduced several new minor and background ponies. Fancypants has become quite popular in the fandom, and his unnamed female companion spawned a bunch of fanart within a day of the episode airing.
There's a funny case with Wonderbolt Fleetfoot. A few days before this episode aired, someone created an O.C. Stand-in out of an Off Model Wonderbolt that appears briefly in "The Best Night Ever", giving her the name Echo. The premise of a blind Wonderbolt was popular enough to get fanart within days... and in this episode "Echo" returns, gets the name Fleetfoot, and wins the race. Said blog adopted it as Echo's surname, and now she's one of the most popular Wonderbolts.
Hilarious in Hindsight: Jet Set and Upper Crust laugh at the idea of Rarity's friends being "important ponies." A couple seasons later, one of those same friends will be an alicorn princess and indisputably one of the most important ponies in Equestria.
Spoiled by the Format: "Hearth's Warming Eve" originally aired before "Family Appreciation Day" and "Baby Cakes", so its plot synopsis was known prior to this episode's airing. As a result, the synopsis that mentioned the *six* friends going to Canterlot to do a play suggested that Rarity would choose her friends over her important business connections.
Strawman Has a Point: With the exception of Fancy Pants, The Canterlot Elite are depicted as smug elitists for treating the ponies from Ponyville as boorish hicks. In addition to Fluttershy ruining the highest profile national annual party, the Grand Galloping Gala (with her friends assaulting several of the party guests to boot), Rarity's friends crash and trash the Canterlot Garden Party (the second highest profile national annual party), making one wonder if the reputation for being boorish hicks is at least somewhat deserved.
A modified take on Ravel's "Boléro" is used during the scene where Rarity dunks Opal in water.
In addition, the music played during Twilight's party is a sound-alike of the 1981 Stray Cats song "Rock this Town."
When Hilarity Ensues, the backing music borrows thematically from Khachaturian's "Sabre Dance".
What an Idiot: Rarity lies to the Canterlot elite about Rainbow Dash, telling them that she's the Wonderbolts' trainer instead of reciting her proper accomplishments such as saving Equestria twice, being the only pony ever to perform a Sonic Rainboom and saving her own life. Her lie about Opal being sick also backfires when Twilight and her friends visit Canterlot to check on her.
The Woobie: It's hard not to feel sorry for the bellhop pony for breaking his back just to do his job.