Was pony Twilight Sparkle not bothering to ask about Flash Sentry or react to his pony counterpart (unlike last time) a sign she's lost interest in him, or was she just too preoccupied with the current situation to pay them mind? How indicative is this of their current relationship?
Was the villains motive that Sunset Shimmer ignoring them showed she didn't deserve her redemption legit? A Dog-Kicking Excuse for acting out of jealousy? Or some of both?
How much of Wallflower's being ignored and forgotten was due to her erasing everyone's memories? Her flashback show her being ignored well before she found the Memory Stone. Was she guilty of not making a effort to get noticed, or was her introduction indicative of how she tried but failed?
Some found the group's ponied-up costumes from Legends of Everfree to be gaudy and overdone. The group gets new costumes in this film that are considerably simpler in design.
One of the complaints towards Juniper Montage and the climax of "Mirror Magic" was Juniper's unnecessary and nonthreatening transformation, and many felt that the shtick of villains becoming Drunk on the Dark Side was getting stale. While the heroines do transform in the climax of the special, the villain does not, becoming Drunk with Power in a more realistic way (by having a powerful, easily exploited ability that they gradually abused more and more), which many fans appreciated.
The premise of everyone forgetting about Sunset's HeelFace Turn and her having to re-earn their trust, as well as being made to more bluntly observe the effects her nasty streak had on everyone, can be seen as this to the side of the fan base who thought Sunset was Easily Forgiven in the initial story arc and had yet to earnestly prove she'd changed.
Awesome Music: The Villain Song, "Invisible", is extremely catchy. Shame it gets seemingly cut short by Wallflower spotting Sunset rooting through her backpack.
Best Known for the Fanservice: Both of sexy and plot-related kinds. The special is probably best summarized and remembered as "the one where Sunset and Celestia reunite, and there's also a Beach Episode with the girls in swimsuits."
Even Better Sequel: While Magical Movie Night was viewed as a good or even great collection of shorter episodes, Forgotten Friendship is still seen as a step up. The stakes feel higher and genuine, the character development is done well, and the scenes of Sunset in Equestria were what fans had hoped to see from an Equestria Girls entry for a while.
Fanfic Fuel: Sunset commenting on how Celestia now has a sense of humor continues to leave the floodgates wide open for interpretation of Sunset's time with Celestia as her student, especially since Celestia's sense of humor is a notable part of her character in the main series.
Foe Yay: Wallflower Blush seemed to want Sunset's attention more than anything else. The Luminescent Blush when Sunset is nice to her at the end only adds fuel to the fire. Also, the way she lashes out at Sunset for forgetting her could be seen as the actions of a Woman Scorned.
One trailer for the special says "love is in the air at Canterlot High" as Sunset and Twilight pass each other and smile. Given that the two are the primary romantic pairing of the Equestria Girls series, and the creators surely know this, this is almost certainly a case of Trolling Creator. Not to mention the special airing only a couple of days after Valentine's Day.
There's quite a lot of interactions between Trixie and Sunset that go beyond mere bonding throughout the story, such as when Trixie confides in Sunset that she feels constantly left out or underappreciated.
Human Twilight gets a little bit more emotional reactions from Sunset than anyone else. Sunset is hit hardest by Human Twilight not remembering Sunset's role in the Friendship Games, and is also the last memories that get stolen from her by Wallflower, crying out "Twilight, don't forget me!" in anguish before they're taken away. When Sunset gets her memory back, the first person she recognizes is Twilight. They're also together in a yearbook photo.
Though to a lesser extent than Human Twilight as mentioned, Sunset's reactions to the rest of the Humane Seven forgetting and treating her coldly show she doesn't take any of it well at all. And at the climax, her selfless Heroic Sacrifice in losing her memories of them and her time in the human world potentially permanently in order to make sure their friendships will still last even if she's not included (and crying out their names as those memories are erased) can invoke a sense of making the ultimate sacrifice for her loved ones.
Princess Twilight and Sunset are rather touchy-feely with each other (albeit ponies are touchy-feely in general) and there's plenty of blushing between the two, most notably when Sunset calls Twilight a "good teacher".
What happens to the Sunset and Wallflower is pretty much the premise of Background Pony.
It's harsh to take Wallflower's motive into consideration when you realize that, outside of Crystal Prep's students, none of the extra characters introduced like the Dazzlings, Gloriosa or Juniper ever make an additional appearance after their first, which means that Wallflower's plight and desire to be remembered and noticed will inevitably be for naught. That said, it doubles as Heartwarming in Hindsight as, now that she has brought herself in the forefront, she can now be used as reference for fan artists and fanfic writers, ensuring that she will be noticed and remembered.
Informed Wrongness: Sunset Shimmer realizes she's at fault for provoking Wallflower Blush's retribution for ignoring them, thus failing to show them she had reformed. But given that she has saved the school multiple times, it's hard to fault Sunset for assuming everyone thought her redeemed. Given Wallflowerunsuccessfully tried to get their attention for a half-hour, they're so hard to notice that Sunset can also hardly be faulted for it.
Just Here for Godzilla: The biggest draw for many fans was Sunset Shimmer and Princess Celestia's reunion, a plot thread that has been left dangling for years.
When the premise first came out, references to Background Pony became widespread. Doubled when it actually aired, with Wallflower and Sunset being "inflicted by the Background Pony Curse" and calling Wallflower's Villain Song the "Background Pony Song".
Editing Wallflower Blush into backgrounds of virtually any scene — even non-pony stuff, like music videos or classic photos — has become a recurring joke due to this movie showing her as being in the background of scenes from previous movies.
Narm: The resolution, where the main characters undergo their dramatic magical girl transformations and manifest a powerful friendship laser in order to... blow up a small rock. Which was in the possession of a completely unarmed student. In a school parking lot. Overkill much? (It doesnt help that the transformation sequence alone is about 10 seconds longer than the actual battle itself.)
Squick: In the extended edition, Trixie happily chows down on a gnawed carrot Micro Chips tries to give Sunset to placate her.
Win Back the Crowd: The works made after Legend of Everfree, while not unliked by most fans, left some feeling Equestria Girls was getting stale. Forgotten Friendship's return to a more adventurous and emotionally driven story renewed a lot of interest in the franchise.
It's hard not to feel sorry for Wallflower Blush, even (or especially) when it's revealed being ignored by everyone drove her to erase everyone's positive memories of Sunset Shimmer in a fit of jealousy. It helps that while she didn't have the excuse of magic corrupting her like some, she was still using magic that she didn't understand the consequences of, and is visibly shaken up when she sees just how painful and traumatic it is for someone to have their mind messed with like that.
Sunset herself definitely qualifies. The memories of her being friends with the others are stolen and her attempts to get them to remember doesn't work. This extends to not just the Rainbooms, but the entire school, meaning she's back to where she started in Rainbow Rocks (but this time without the Equestria Girls on her side to support her). Then, she has her own memories of her time in the human world stolen and is scared out of her mind at suddenly being in an unknown place and body. Anybody else wanna give her a hug after this?