SPOILERS, as if it needed to be said. Read with discretion, if you care.
I'll say this. "Magical Mystery Cure" is not a bad episode. It is however a very, very, very, VERY, VERY, VERY incomplete one.
Let's start with the summation. In Ponyville, it appears that the other members of the Mane Six have been switched around in their occupations and lifestyles because of their Cutie Marks, the indicators of their destinies, being swapped. Naturally, this causes some depression and stress among them, like Fluttershy becoming increasingly terrified at the unamused crowd she's trying to entertain while bearing Pinkie's mark or Rainbow Dash being subjected to the ol' "bound in a cauldron" gag by animals she can't tame like her Fluttershy's mark implies. Twilight is horrified by this switch, especially when she learns that it was her fault. (She had received from Celestia a spell created by Starswirl the Bearded-who's the quintessential posthumous wizard character-that was incomplete. Being the student she is, Twilight recited it in an attempt to decipher what it could mean by "destinies" and all...within proximity of the Elements of Harmony.) Throughout the episode, she sets out to right the wrongs by essentially reminding the characters of their true passions-be it having Fluttershy talk to the animals or Dash fix up the checker-patterned clouds-all to a conclusion where the incomplete spell's natural end is that the key is (rather predictably) friendship. And, since we all saw this coming, this is revealed to have been the final test conducted by Celestia as the final step in Twilight's ascension to alicornhood.
So...that's all it takes? Ikay, let me get my misguided friend to their workplace after they were given faulty directions. Where's my wings?
I'll say this, I do like the premise of the first half of the episode. In a series that builds around destinies and true passions and stuff of the like, it's fascinating to address the concept of being pushed into a painful situation because it seems as though destiny is telling you do so. Although I personally have never seen the Cutie Marks as a marker of some twisted deterministic worldview and see them more as indicators of true passions, the conflict is still valid. We see how these characters react when they feel pushed to betray their true passions because some indicator says you have to abandon your animal care abilities because it says you have to entertain. There was a genuinely interesting element to seeing the characters' reactions and such and some sadness felt seeing them try so hard to do something because some indicator says so.
But here's my first major problem with the episode: it's not explored enough. Due to it being a Twilight-centric 20-minute episode, we really don't see much of the other Mane Six aside from small snippets showing the character singing of their frustrations and despair. How did they come to this conclusion that because they've one Cutie Mark different, they're meant to be like the other? Did they fight against it before succumbing? Was there more depression and inner turmoil? What more could be said about their situations? This isn't explored enough. There's enough material here to be make it worthy of its own episode, if not a two-parter. But they don't explore the ramifications of a mark switch in a universe that is built around discovering one's true passion. (Yes, I will call that as opposed to "destiny". Although if you want to use the classical definition of "destiny", it's more like what will happen at the end, not throughout the journey.)
This segues into my next problem with the episode: it feels like they were attempting to condense a much larger storyline. Seriously, the episode feels like a TV musical special that they crushed to fit in the perimeters of the 20-minute story. I have to ask why. Like "Keep Calm and Flutter On" (the reforming Discord episode), this story was rushed and not allowed to flesh itself out more. Honestly, this story really suffers because of the decision to cut it short. It feels like I'm watching an non-indicative "...in Five Seconds" vid trying to cover everything within a small amount of time. No, for a story like this, it should have been expanded. Explore the afflicted character's new lives made all the harder by this apparent change in destiny. Really get into the whole "destiny" issue the series likes to bite at with its talks of Cutie Marks and such. And, honestly, strengthen the solution. While the literal "Power of Friendship" message isn't delivered too poorly because it actually works with the message of friends emphasizing their support network, it's too immediate and, admittedly, obvious. So basically, the whole issue with true passions and destinies is because you lose sight of your friends? Alright, if you say so.
And it might strike some as a bit dickish that Celestia would give Twilight a spell that, for all intents and purposes, messes up her friends' lives, for the sake of a final test. Um, wouldn't it have been better if Twilight discovered the spell herself and thus acted to save her friendship and fix a mistake she made-as opposed to a deliberate test that briefly left her friends in depressed states? That honestly could have made for better character and plot development, showing that Twilight really has the qualities of being a leader without it being some cosmic BS. (And if you want to get into that rhetoric, so it becomes more about her actual talents and character rather than a path set by conjured destiny.)
That said, not a bad episode-just so painfully incomplete and unexplored. And as I've said before, while I partly disagree with her ascension, there is storyline potential and new possibilities to explore. And I guess if all fails, there's always the IDW comic.