I'll have a more detailed review up later, but suffice it to say, while it struggles to stick the landing, this is easily the best Epic Story the show's done since "A Canterlot Wedding", maybe even since the original "Friendship is Magic" two-parter. I'm VERY satisfied with it, more so than I've been with the show for what feels like ages.
It does officially mean "Escape From Castle Midnight" is basically an AU, though. But that was always a possibility, so I'm not crying over it. I have my OWN epic story to tell, and I only hope it lives up to the standard "Princess Twilight Sparkle" set.
EDIT: OK, had some time to chew it over, and I think I've got my thoughts more properly arranged at this point.
In the broad strokes, I think I may have been a little over-enthusiastic in my initial assessment; this is good stuff, and my overall list of complaints is shorter than they've been for the last several Epic episodes...but the complaints I do have are a bit more critical than usual. It's still a significant step up from "Magical Mystery Cure" or "The Crystal Empire", to be sure, but that final third runs into some serious problems that bother me worse and worse the more I think about them. That said, it also contains some of my favorite moments in a long while, and the overall arc of the story definitely worked for me. There were a lot of strongly-executed surprises in this one, and a sense of genuine scale and danger that the show's been missing for a long time. I was impressed as well by the overall animation; the show's overall visual style has gotten better and better with time, and this two-parter showcased a lot of those new strengths.
So, as per usual, it's not perfect, but it is quite satisfying.
And now, for the more SPOILERY specifics of my opinion...
* I was genuinely impressed at just how deep the world-building and mythos-expanding went into this episode, even given the show's surprising willingness in the past to do just those things. More importantly, WHAT elements of this show's history and world they chose to expand were well-chosen and excellently executed; I never would have expected to actually see the fight between Nightmare Moon and Celestia from 1000 years ago in detail, but it was a genuine treat (and a superbly-done scene, too!) even so. Learning more about the Elements of Harmony, and further learning that we in fact still have a lot more left to learn, was also quite unexpected, but very appealing.
* Discord was a hoot-and-a-half all throughout the episode. His reform is sincere, sure, but that doesn't mean he isn't still something of a brat, and it was nice to get a clearer impression of just who he is and where he stands post-"Keep Calm and Flutter On". His dynamic with the Mane Six was a lot of fun to watch again, with lots of great little moments (such as his reluctantly helping the townsponies or shrinking down at Fluttershy's scolding) that helped flesh out his character nicely, and the way he does, in fact, help-but does so in his own particular, Discord-y way, was quite clever and enjoyable.
* The overall thrust of the story was really well-done. The sense of fear and confusion that fills the first half is pretty powerful, and the delving into the greater past of the Elements that takes up the sescond was likewise great. I think my favorite choice, however, was to have the Everfree Forest itself be the closest thing to an antagonist the episode has. I know some fans might be disappointed we don't get to add another Big Bad to FiM's surprisingly-robust roster (yet), but after Sombra and Sunset Shimmer both failed to fully work as Villains, I think the choice to go for something more primordial as the central threat was a smart choice, and one that yielded some strong dividends.
* Lots and lots of fun little Character Moments. Pinkie's drooling over dreaming, Rarity's boutique problems, Fluttershy's Discord Control, Rainbow being...well, Rainbow about everything, AJ being take-charge (though that feeds into my Big Problem with the episode, but we'll get to that later; for now, let's focus instead on all the awesome lassoing and stepping up when needed), and of course, Twilight Sparkle coming to terms with her Princesshood. That last one, I admit, took a bit of time to fully appreciate; after all, we saw Twilight flying as confident as could be at the end of "Magical Mystery Cure", but here she's so bad at it she's better off walking, while it initially felt like "Equestria Girls"' lessons were being swept under the rug. Thinking it over more, though, the episode's choice of how to further explore Twilight's transformation worked for me; "Equestria Girls" was chiefly about her learning to accept her Leadership and the fact that these changes will take time to adjust to, and this episode shows both those things in action; she is neither afraid nor unable to take charge when Celestia and Luna go missing, after all. This episode's focus is, in fact, on what she doesn't have to change: who she is, to herself and to her friends. It suffers a bit in execution (but again more on that later), but overall it definitely clicked for me.
* Indeed, the choice to return the Elements to the Tree at the end was not only unexpected, but brilliant; it feeds wonderfully into the episode's overall themes of Change and the nature of Power and Friendship, and made for quite the bit of visual beauty.
* Pacing. This has been the big problem with the last couple Epic episodes for me, but "Princess Twilight Sparkle" nails it. Unlike "The Crystal Empire", which puttered around for two-thirds of its running time, or "Magical Mystery Cure" which raced through every plot point like it was on fire, "Princess Twilight Sparkle" is able to set up, explore, and resolve its central conflicts at just the right speed, and it makes for a more satisfying experience.
* Oh, also, the new intro is great; all the new little touches they've added really work and I love it. <3
* I only have two noteworthy problems here, but unfortunately? They're kind of biggies. The first one, and the less harmful overall, is that it feels like the characters are a bit out of focus here; the episode gets a little too wrapped up in all the world-building and History-visiting, to the point where the emotions and dynamics of the characters, the things that ought to always form the heart of the show, wind up feeling a little kept to the side. Zecora in particular comes in and out of the plot with very little room to breathe, but as a side-character that's fair enough; still, I would have really appreciated more time spent with our girls working through the emotional effects of this journey, because the moments when we get to see that are by far the most potent (that opening scene, for example, of Twilight airing her worries to the girls about what being a Princess will mean to their friendship), and they are sadly a bit too thin in the episode itself. There are some other focus issues(the fact that unicorn magic is on the fritz is raised at the beginning but winds up kind of fizzling out immediately afterwards; I feel like its impact ought to have been more keenly felt), but in comparison they're pretty insignificant.
* I have a really, REALLY hard time swallowing the Final Act twist of the girls sending Twilight away as it is presented here. Maybe, if it had been more about their wanting to keep her safe as a friend than as a Princess, I could have bought it, but the way it's framed here rings patently false (and muddled, as Discord acts as if their choice was borne more from their friendship than Royal duty); I get that it's meant to feed in to the overall moral about how Twilight's new powers and Princesshood cannot ultimately change her friendship with the girls, and the episode aligns itself significantly by using this not as an opportunity for Twilight to bail her idiot friends out of trouble again but rather for them to realize their mistake and come to her rescue instead, but right now it is easily the biggest sore spot in an otherwise solid episode. I imagine, with more time, I might come to see it in a more positive light, but right now it sticks in my craw something fierce.
Still, on the whole, "Princess Twilight Sparkle" is a satisfying start to the new season, one that pretty openly sets us up for further great adventures to come without sacrificing its own integrity as an overall story. So here's to Season 4; we're off to a good start, and I imagine it's only going to get better from here.