Finale...

Oh my god. The songs kept me constantly with goosebumps (which tends to happen whenever I really like something). It's the first episode since "Keep Calm and Flutter On" that has kept me interested through the whole thing. Despite the fact I felt the cutie mark thing could have been played with more, it was paced well and kept characterizations fairly well.

A few things in particular I liked:
1) Pinkie doesn't have a bad country singing voice.
2) To expand on the above, I like how the music fit the settings of the characters for the "Cutie Mark Song" but the voices themselves provided dissonance and didn't fit with the music types.
3) I've kind of waited for a musical episode for a while, and I think Ingram and Larson pulled it off well; the songs were enjoyable and rather catchy to the point where I was singing along with "What My Cutie Mark is Telling Me".
4) The ethereal space right before the transformation.
5) Celestia sings. And she does such a damn good job I've wondered why they didn't have her sing at least once earlier.
6) At the end of the episode, Twilight seems to have maintained her adorkable self.
7) Given how well this episode was pulled off, I can't wait to see what Season 4 has.

A few things in particular I didn't like:
1) The whole thing with switching cutie marks... ended a little fast and could have had more into it. It felt like it was solved at a fairly large emotional high for a part as early into the episode as it was. For example, it could have had a point where Scootaloo, Sweetie Belle, and Apple Bloom were wondering "What the hay is going on with our sisters?" when we only see Apple Bloom and only when Applejack is restored.
2) "A True True Friend" and "What My Cutie Mark Is Telling Me" were probably the two songs that, for me, had a decent length. Some of the songs had the potential to be much longer or felt as though they ended early; Twilight's "I've Got to Find a Way" and "Celestia's Ballad" to me felt they could have had one more stanza and Celestia's in particular felt like it cut off rather quick.
3) I like how Celestia acknowledged being reunited with Luna during the series premiere, but I (with something of a bias) think that Luna should have had at least one line on the matter... or in the episode for that matter.

Also this episode contains my two favorite lines from the show now.
1) Pinkie: Hold on a second! takes a drink of water, only to spit it out
2) Twilight: to Shining Armor Are you crying?
Shining Armor: crying No, it's just a bit of liquid pride.