"Wait, so is this episode about Twilight fixing her friend's cutie marks, or about her becoming a princess?" "Both!" "How can you possibly fit both into 22 minutes?"
You can't. I'm just going to list what happens to try to figure out WTF was going on, because I don't even know what the heck I just watched.
So, um, I guess Twilight screws up a spell -- she said she tried to cast the spell although it didn't actually show her casting anything. I guess she read the 'spell' out loud and that set it off? Even though that's now how any previous spell in the show has worked ever?
Anyway, it shuffled her friends' cutie marks around because of the elements of harmony or something, and changed everone's memories so that they thought things had always been that way even though they were all horrible at their new jobs. Because we're really in a hurry, instead of her trying anything that might work but doesn't, Spike lists the options verbally instead and Twilight shoots them down instantly without explaining why they wouldn't work.
"A memory spell!" "No!" "That cure for the cutie pox?" "No!" I think there were a couple others.
Instead, she tells them to go help their friends do their job and because they're such good friends they humor her. And it works! They all get their cutie marks back. Twilight is inspired by this to come up with the second half of the rhyming couplet in the back of the book, and because she didn't learn her lesson reads the poem out loud AGAIN accidentally casting the spell AGAIN and this time it kills her.
In pony heaven, Princess Celestia shows Twilight a hilite reel of the series and says that Twilight is awesome and has fulfilled her destiny, then sends her back to the world of the living where she has angel wings and for some reason is now a princess instead of a zombie/vampire/angel/whatever. Her friends are surprised and happy to see her alive, so they just go with it.
Then there's a five-minute coronation sequence during which nothing happens.
This is like the cheesy ending to an anime series where they don't really have any idea how to end it so they just throw in a bunch of flashy transformation junk, isn't it.
I'm really glad there's going to be a season 4 because this would be the worst way to end a series ever, aside from I guess every anime series that ended the same way (I'm looking at you, Hyper Police!)
It wasn't as bad as the previous episode though because while it was incomprehensible, it at least wasn't painful to watch. The songs about the cutie mark disaster were pretty cool, it was just that everything that should have gone between them was missing. And the coronation princess crap etc. was just tedious and not actively bad.
This really should have been a two parter. There was even the normal break in the middle where the tone completely changes that the two part episodes have -- you know, how it has *two plots* which could fill *one episode each*. And a cliffhanger in the middle to make sure you want to watch both.
The episode would have been SO MUCH BETTER if they'd just... I give up. v.v
EDIT: Okay, un giving up a bit, to say -- the first part of the episode was actually good and you should watch it. Especially Rarity. Rarity was the best pony in this episode closely followed by Pinkie. They tried hardest!