Celestia's maniacal laughter is the last freakin' thing I ever expected. It was so out of left field, so played-to-the-hilt, I'm still not even sure how I feel about it. I think it was great, but damn if they almost went overboard with the mandate of suddenly giving Celestia characterization at long last.
The story was laudably out-of-formula for a premiere and the Twilight imaginationland scenarios (and Spike as the peanut gallery) were excellent fun, but I feel like I missed some key thing about the plot somehow. The story beats as I recall them were something like:
* Discord sets up the conflict by forcing Twilight to have to think of a new plan for Starlight's future
* Twilight flounders through several self-induced false starts of possibilities involving sending Starlight on assignment to some remote place where she would probably (according to Twilight) get into trouble, and all of which have the common theme of "she'll be there for a hella long time"
* Celestia goes HAW HAW in Twilight's face, says "Hey you giant flying purple nerd, you should let go and not try to control Starlight, you're basically family now and nobody likes a helicopter mom"
* Twilight decides to ... just tell Starlight "Congrabulations, you graduated, you're a full-fledged tank paratrooper" and not assign her to do anything
* Everyone cheers
Which... I
guess is coherent, but it also feels like something of a cop-out on Twilight's part, doesn't it? Isn't that just the easy escape hatch, to spring a "graduation" on her that she didn't even realize she was working toward? Like a guitar tutor who tells Dwayne "It's all in your fingers now, I have nothing more to teach you", you expect that he's saying that not because he literally thinks Dwayne is his equal in skill, but because he's just sick of trying to provide structure and discipline for someone who wouldn't benefit from it anymore?
I'm not sure I know what central point I was making to the princess man here. I guess I was expecting Twilight to come up with something a bit more ... ambitious to do with Starlight than to just wash her hooves of her and say "go forth and prosper" and sit back in satisfaction at a job well disposed of while making this expression:

Maybe what threw me off was that Celestia's story of sending Twilight to Ponyville seemed to be telling her that the right choice
was still "You have to send her to some place with an important political assignment", but Twilight interpreted that story instead to be about letting go of control, and that was the point. I think the way my mind took it was to think that Twilight heard "Celestia sending me to Ponyville wasn't about defeating Nightmare Moon at all, but rather was about meeting the other five, who apparently all knew each other and were all good buddies even before I got there"—complete with narration drawing explicit parallels between Celestia fretting about the dangers Twilight would face and Twilight fretting about the dangers Starlight would face—and instead of keeping to the parallel and finding a Ponyville-like assignment to send Starlight on, Twilight only heard "You need to let her fly away, if she loves you she'll come back".
I guess that's a complete story after all, and I don't know what I'm on about. Maybe I shouldn't have taken Celestia's fourth-wall-cracking one-liner about "You can always make her write you letters" as merely a hilarious gag and realized that it was actually the key thing I was missing at the beginning of this post. Twilight was better at catching the point of Celestia's story than I was. I would make a sucky princess of friendship.