These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: MLP: Friendship Is Magic S2 E25 "A Canterlot Wedding - Part 1"
If you're feeling uncertain, jealous, or downright hostile towards someone who's about to marry into your family, you are absolutely right, and it's not simply a matter of you making a poor transition.
On the other hand, Twilight is shown to be overjoyed about the marriage until Cadance misbehaves. Could be a much more valuable aesop to kids: don't ignore someone's bad behavior that occurs in private just because they make your family member happy.
Broken Base: Even before The Hub's premiere, a few fans called Executive Meddling on this two-parter, since it concerns a frilly, girlish wedding of two characters only introduced this episode that no one has ever heard of before for the sake of Hasbro's marketing, while other bronies are calling it a fitting way to end of the second season and will watch it on TV anyway.
Twilight turned out to be right about the Cadance we saw and had anyone listened to her, a disaster would have been prevented, but did the rest of the Mane 6 and Celestia really forget the aesop of "Lesson Zero" or were their reactions justified by how Twilight had been acting lately? Fans can't decide who the bigger jerkass was. However, there's also another group that just wants both the defenders and detractors of Twilight's friends' actions to get over it and move on, since, in the long run, it's a minor plot point compared to the others that happened in the two-parter.
Even among those that enjoyed the episode, a common adjective used to describe it is "fanfiction-y" because of the Remember the New Guy method of introducing Shining Armor and Cadence and latter doesn't look out of place in a Deviantart search for Alicorn OCs. You'd think with all the trips to Canterlot in season 2 Twilight would've encountered or at least mentioned her brother once, especially with how clingy she appeared to be over him.
Let alone that you'd expect someone so important to her to be present at her entry exam for the most prestigious school in Equestria.
Cadence being an alicorn, and how this interferes with previous interpretations of Celestia and Luna. It doesn't help that, in this two-parter at least, her being an alicorn is pointless to the story.
There's some that argue that Twilight was Right for the Wrong Reasons, due to the behaviors that set Twilight off, and later turned her out to be right about the evil Cadence all along despite having reasonable explanations (though Fluttershy's explanation of one of her birds sounding off-key turns out to be true). True, she never quite identified the spell used on Shining Armor, which may have given him at least temporary relief from his headaches, and never tried to figure out why she was acting like that from the start, and didn't pick up on Cadance not recognizing her, which was vastly more important than anything else Deceptive!Cadance did and you can argue that this makes her friends brushing off her accusation more understandable, but however you make of it, it still counts that Twilight was indeed right about "Cadence".
Contributing to this, there's even those who insist that if Twilight had explained herself better prior to bursting in at the rehearsal to declare Cadance "evil" and gathered evidence to back up her accusations, the others might not have brushed off her concerns so easily. Princess Celestia in particular had never once doubted her before, so to them, it shows how much Twilight apparently screwed up. Ironically, in this case, this may have worked better in the long run because it caused the villain to reveal herself unambiguously, so it appears to be somewhat Played With. Contrariwise, there's those who insist that there was no way Twilight could have collected any evidence in this case.
Foe Yay: The Incest Yay Shipping for Twilight Sparkle and Shining Armor gets more shippy after the latter calls the heroine out for saying that Cadence is evil. Watch that scene as many times as you like. It would soon start sounding like a Sibling Rivalry conversation.
Harsher in Hindsight: The need for increased security at the wedding seems a more eerie in light of the issues with insufficient security at the London 2012 Olympics.
Cadance is ironic because of Hasbro's pink Celestia's toys.
Awhile back, there was a "Smug Cadence" meme which had a picture of the bride-to-be with smug face making snarky comments about her being another alicorn, her being pink, and her marrying Twilight's brother among other things. First episode airs and Cadance is shown to be very smug and snarky. In truth, the imposter was and the real deal isn't.
Cadance in some people's eyes. Her being an alicorn is the major argument for it but there are other factors common in "Pony Sues": a gradient color scheme, royalty, looked up to and admired by main characters (Twilight Sparkle has admired her since childhood), becoming adored by everypony without any effort which she does once released and able to take part in her wedding preparations, having powers not previously indicated by the show (not a major offender but it is common to such characters), and having the big day-saving scene center around her actions. On the other hand, the following episode showed that she was soundly defeated by the Hive Queen and is now nigh-powerless, and also decidedly NOT any more perfect than any other character.
In-universe, Imposter Cadance is a Grade-A Jerk Sue; Twilight can't understand why her friends are defending Imposter Cadance's ridiculing their passions and what her brother sees in her.
Strawmare Has a Point: If you want a mature and dignified royal wedding reception, Pinkie being an arranger makes just as much sense as hiring Pee Wee Herman for any of the British royal weddings.
More to the point, Cadance says Pinkie's party ideas are only suitable for a six year old's birthday party (balloons, board games, and barn dance music), and this is pretty much the ONLY type of party Pinkie seems able to put together... Perhaps Celestia wanted to have some fun like at the Grand Galloping Gala again?
Granted, the real Candence was shown at the end to be enjoying the games that Pinkie came up with. Given that it was the imposter who didn't enjoy them, maybe Pinkie knew what she was doing, after all.
Tear Jerker: The entire ending, from Shining Armor revoking Twilight's position as Best Mare and banning her from the wedding, to her friends and mentor heartlessly ditching her, to Celestia's ice cold remark: "You have a lot to think about." Made all the worse when Twilight turns out to be in the right after the fake Cadence imprisons her underground. Just put yourself in Twilight's place and, despite feelings of jealousy and not showing any proof when you rightfully accuse the imposter, try not to feel the pain, or in the case of the Broken Base, utter hatred towards those who left her.
Twilight when she straight out accuses Princess Cadence that's she evil during the wedding rehearsal. What was she expecting?
Why Twilight's friends, family and even Celestia, of all ponies, had suddenly forgotten that Cadence had been acting like a jerk to them, and was Not Herself today, especially after Twilight says that she is evil. This might be mitigated by the whole "getting stressed preparing the wedding" thing.
The Woobie: Twilight. Dear Celestia, Twilight! Not only did her brother ditch her, but so did her friends and mentor... and then she turned out to be right!
Jerkass Woobie: She probably shouldn't have been acting jealous towards her brother in the beginning, but it still doesn't justify her being alienated.