The main thing, of course, is that nightmare moon wins. The key scenes that came to me:
Celestia casts the banishment spell when Nightmare Moon appears, using the same terms as the first time she cast the spell: "1000 years, or until you forgive me". Nightmare Moon panics even though she's sure that without the Elements of Harmony to boost it, Celestia's spell can't possibly work. "I assure you, the spell will work," Celestia replies. Then she banishes herself. (because otherwise NMM was planning to destroy her)
Applejack, the farming baroness, and her accountant Twilight Sparkle (whose cutie mark is just one of those *crazy* star patterns that unicorns from Canterlot tend to have and not representive of an actual explosion of uncontrolled magic that nearly destroyed a cider bottling plant THANK YOU VERY MUCH) demand an audience with Nightmare Moon to explain why they need the sun back or everyone is going to die. Twilight brings lots of charts and graphs. Applejack brings her stubbornness.
NMM's honor guard is Rainbow Dash and Fluttershy, as shadowbolts. Rainbow Dash (a down on her luck loser with no particular ambition) was the only civilian to actually attack her when she appeared at the Summer Sun festival, and Fluttershy is, well, Rainbow's friend (who, among other things, nursed her back to health after she attacked NMM at the summer sun festival). "This costume makes me feel so assertive!" It's a ceremonial position anyway; the real Shadowbolts are NMM herself in disguise.
Pinkie Pie and Rarity are there as a court jester and a courtier respectively. Those aren't their official titles; they run a gem mine together and nightmare moon likes shiny things.
NMM refuses to back down on raising the sun because it would mean admitting Celestia was right. "The sun was a crutch, making you weak and beholden to your 'glorious princess'. You little ponies will find a way to live with the eternal night or I'll replace you with creatures that can!" Then she banishes everyone from her sight. Including her guards.
Rarity decides that this is a royal commission to develop a replacement for the sun, with her in charge of course. She delegates actually coming up with an idea to the others. Twilight proposes the idea of using rainbows to illuminate the crops, but rejects it because it's too expensive. "The number of gems it would take to permanently enchant a rainbow would make it completely impractical." Applejack thinks that makes it a wonderful solution -- expensive crops means a higher profit margin after all. Since most of the expense is on gems, Rarity likes it too.
So, they build a prototype and go to test it out on a farm near NMM's castle, Sweet Apple Acres. Big Mac is not especially happy to see Applejack again. They have a dozen unicorns working together to try to enchant the gems, but they're not strong enough.
Applejack: "You could do it, Twilight."
Twilight: "No. Nonononono. That's a terrible idea. I don't do magic. I'm not a magic unicorn! I do math. Math and books. I like books. Nice, safe, books that don't explode and turn people into cactus --"
Applejack: "You'll be fine. We'll just get another unicorn to help guide the magic..."
Rarity volunteers to stay close and help guide the magic (since none of the professional unicorns want anything to do with it).
Fluttershy engages passive aggressive mode to remind her how much they really really need this to work. "Or, you know, we could give up on this and just go try something else even though we didn't have any ideas and we already spend a week on this, and all the poor ponies and animals will starve."
Pinkie Pie mocks Twilight for being so afraid of losing control of her magic that she gave up on magic entirely. Twilight starts listing off all the horrible things that might happen to anyone nearby and Pinkie laughs and says it sounds like fun.
Then Rainbow Dash, of course, startles her into actually losing control of her magic with a lightning strike. Twilight floats up into the air, eyes glowing, all the ponies around her levitated up off the ground and caught up in the magic... and Nightmare Moon, watching, realizes they all embody the elements of harmony and panics, trying to dodge the rainbow but it's NO USE! So she swoops down and smashes the emitter before it can... well, actually, after it hits her full on and has no effect. It was just a rainbow.
Twilight (in panic mode): "It's okay. It worked! That's the important thing, right? We needed to make 2,736 of them to actually save Equestria anyway, so losing *one* isn't really a setback."
Applejack: "How many of these could we make with the gems on hand again?"
Rarity: "Two. Well, two and a half. Maybe three if we use all the cut gems we have in stock, and disassemble all the jewelry."
Then Fluttershy does her "You BIG. DUMB. MEANIE!" thing at her and gives her the stare, and goes off on a rant about how they worked so hard and if she was so paranoid that they were all out to hurt her, even though nobody was trying to fight her except in her own mind, that she wasn't even going to let them make harmless rainbows then she had to bring up the sun. Right. Now.
And then NMM repents and brings up the sun and Celestia comes back and they all live happily ever after. "You planned all this, didn't you sister." "No, actually, I was planning to blast you with the elements of harmony again, but I couldn't actually get the ponies in place to use them in time."