A few thoughts that I had while watching the awesome finale:
1) Tabitha St. Germain does a pretty good job giving Rarity and Luna different voices, but during the singing part, I realized that maybe they should have looked for a different singing VA for Luna (= not Kazumi Evans), because I think no one did not notice how Luna's singing sounded identical to Rarity's singing.
Hell, if nothing else worked, maybe they could just have done some careful pitch-lowering in post-processing. It just felt way too weird to hear Rarity sing from Luna's mouth.
2) Did you notice that Luna was uncredited for a long time? I'm not sure right now, but the first time her VA appeared in the credits in connection to Luna might have been in the beginning of season 3.
3) It took Twilight a while to think about Discord's bookmarks (it was obvious to me right away), and it makes sense considering she didn't really accept him as a friend, and then she said that she hates to admit that Discord might have been trying to be a good friend after all. ... What the hay? Now how is that meant?! Why do you hate being wrong about your belief that Discord hasn't changed? Shouldn't you be happy that you might have been wrong, hm?
Just as you should be happy when you were wrong a moment later when you told Pinkie regarding the key and the box "I don't think that's gonna work." and it did work.
Twilight Sparkle didn't believe in Discord's change because she had trouble with changing herself.
But she is/was also delusional, because while sometimes she is open about her resentment for Discord, at one point in the finale she said:
"How can he do this? I thought our friendship meant something to him."
This sounds quite arrogant. She merely tolerated Discord, like an inconvenient necessity to keep in check, and then she says something like that.
All I can say is that I have witnessed this degree of trouble comprehending basics of friendship and sincerity in people, so I'm not saying this is unrealistic. It's just that I'd have expected better from Twilight Sparkle. ... Hm, then again... I sometimes say the same about real life people.
So Twilight said: "I thought he had changed". But her attitude towards him didn't change that much.
Someone once told me something I'd call remarkable spiritual wisdom that's worth exploring:
People change once you change your perception of them.
This ties into the next point...
4) Some people say they expected Discord to be playing Tirek for a fool, but not me. I think he did the betrayal because he didn't experience much friendship from the mane 6 except from Fluttershy, so his old life was still enough of a temptation. He felt like Fluttershy was merely providing him an anchor of friendship in an otherwise unaccepting social environment.
If anyone deserves the title "Princess of Friendship", it should be Fluttershy. I empathize a lot with her regarding this matter, and had I been around in Equestria, I'd be the best of friends with Discord, especially in tandem with Fluttershy. (But maybe this is also because of my Pinkie nature.)
5) Then there was that huge plot issue that was like the absolute low point of the finale writing:
Virtually everypony in Equestria knows about Alicorn Princess Twilight, yet Celestia thinks she can 'hide' the fact that there is a fourth princess and have their magic safely tucked away in Twilight.
The huge blunder with the stained glass window was just the icing on the cake that further underlined how she totally didn't think this through.
6) There was a remarkable scene where Discord was robbed of his magic. He mixed his old ambition with his new virtues and thus put trust in a villain, as Fluttershy would have done.
It is then very nothworthy and surely fully intended by the writers how just like Fluttershy a moment before regarding Discord's betrayal, Discord, once betrayed by Tirek, weepily said the same thing, that he didn't see it coming. Identical dialogue. This underlines how much influence Fluttershy as a true friend had on him, while he was lacking the strength and virtue of the friendship from the other five ponies, thus he probably sought it from Tirek.
7) The end credits state Tabitha St. Germain as the VA for Photo Finish. Where did Photo Finish speak? I didn't notice that.
8) It would have been cool if the alicorn-depowerment left them looking somewhat different, Luna for example in her season 1 look, and Celestia maybe without her mane multicoloring. Also, while it was probably impossible due to lack of the necessary writer foresight, it would have been neat if Luna's flank speckles turned out to be something she got due to her Nightmare Moon phase. (But we already saw a kind of 1000 year flashback where she had them, so this would have been a contradiction.)
Oh well, speckles aren't anything uncommon for horses, so I guess it's fine.
9) And now... I invite you to check out the 12 caption pictures I made to express various other brief thoughts that came up during watching the finale as well as hunting for good frame captures. I didn't link the pictures here directly because the descriptions are part of that style of my commentary.