So, my thoughts? Not the strongest story ever, perhaps; it had a pretty hectic beginning with no time to properly ease into the story, the plot was relatively straightforward, and setbacks and deviations from the straight line (such as Twilight's facing her worst fears at the enchanted door) were as short-lived as they were few and far between. Nonetheless, it wasn't bad, and it certainly made me very happy to see the crystal ponies saved.
Speaking of crystal ponies, I'll freely admit that the story seemed, at least in part, motivated by a desire to get more toy sale opportunities. Whereas other shows where the show itself is the sole (or at least main) product might focus on the established dramatis personæ and add depth to their characters, their background and their interactions, the focus was obviously on providing something new here; and except for Twilight (and Spike), most of the characters, including the rest of the Mane Six, Shining Armor and Cadance didn't have much of a chance to really shine. The one I was least happy with was Pinkie Pie, perhaps: even though she represents the Element of Laughter, I think there's more to her than whackiness, fourth wall-breaking and comic relief.
King Sombra, similarly, also wasn't the show's best villain; Night Mare Moon had hints of a deeper back story that's yet untold, Discord had character in spades, and Chrysalis had a motivation that made you consider just how bad she really was (even if her methods were unacceptable, her goal of looking out for her changelings and finding food for them was surely noble). But Sombra? For the most part, he just seemed evil for the sake of it; OTOH, one does have to wonder how he turned the way he did. They DID mention he used to be just a regular plain old unicorn, after all: and given that the crystal empire disappeared a thousand years ago, one also wonders if there was any connections to Night Mare Moon's rise and banishment.
The framing story with Twilight being tested by Celestia also suffered from a lack of time, and Twilight's fear of having failed because she didn't follow her mentor's instructions down to the last letter and instead opted to follow their spirit instead seemed a little strange. OTOH, it IS a show for kids, and for kids it's an important lesson to learn: don't lose sight of the forest for the trees. FWIW, I also hope that the whole "Twilight's gonna move to the next level of her studies" bit will keep the show strong and fresh and allow it to portray genuine character development and change, as opposed to getting stuck before it gets started, a fate that befalls far too many shows.
Celestia was gorgeous as ever, and showed again that there's more to her than many, even bronies, seem to be able to see, and I couldn't help but think what a heavy cross to bear hers must be. Luna, meanwhile, had some echoes of S1 Luna again, at least visually, but then she only had a very small part. (It's funny how this precise thing was spoofed in the S2 ending before being shown, seemingly in all earnesty, in the S3 starter.)
One thing I loved? The music, of course. I found myself singing along "I wasn't prepared for this" almost word by word (the first time, anyway, not the reprise). :) And of course there were many moments that gave me a good laugh or made me smile, many references (windy mane Twilight!) and throwbacks; one of the best lines was Rarity's "I had to make a hat out of three strands of hay and a drinking straw! ... I made it work". That was Rarity in a nutshell, ALL of her: fashionable, hard-working, dedicated to a fault, and also just a wee bit of a showoff, a pony who delights in being the center of attention (and admiration).
I also liked how Shining Armor displayed a knack for quick thinking and, shall we say, creatively unorthodox solutions. :) That, and a good throwing arm. And Twilight's use of the anti-gravity spell to deal with those stairs was also pretty neat. (I'm still wondering just how magic works in the MLP:FiM universe in general, BTW, although it being similar in principle to how it worked in AD&D, especially in the Forgotten Realms does not seem to be such a bad assumption.)
What could've been done better? I mentioned above that the episode seemed rushed at times, barely managing to fit into its ~44 minutes, but for that it spent quite a bit of time showing the various activities at the crystal fair and the crystal ponies' reactions to them. I'm not complaining there, but I might've cut a bit more there myself and rushed things a little less elsewhere — but it's always a tough call to make, and perhaps two episodes just weren't enough for it all no matter what.
All in all? A very solid two episodes and a good opening for S3. I'm really glad that there's new pony content from Hasbro, and I'm very much looking forward to the rest of the new season. Kudos to all involved! :)
...BTW, did they ever explain what was up with Shining Armor's horn?
EDIT: I'm not the only one who felt this way about the villain; see e.g. this or this (spoilers!).