EDIT: To Cloudane, just saw you're post, I'll respond directly to it when I can. /)
Okay everybrony, here's my basic opinion on the episode in a positive and negative formation;
Positive Points
1. It was a beautifully written episode, and was a good character developed storyline. I wondered how an episode about cutie marks switching could even work as such an important episode, even with my fascination with cutie marks, but it was a real eye opener; this episode shows that you're cutie mark really is about "who you are", and if they were wrong it would be obvious. I don't think it's possible to get the wrong cutie mark the normal way, but it really showed that the characters in this show have a personality and are not one-dimensional, they have strengths and weaknesses, and this episode really showed that.
2. The songs were just beautiful, and didn't get in the way at all - I was expecting to not like at least one, but it added so much to the show, and some of them made me really believe what was happening.
3. The scene where Twilight is shown scenes from pervious episodes to show how far she's gone was just heart-warming and even a little tear-jerking.
4. I actually do think that maybe, Twilight becoming a princess was planned from the get-go. Think about it, there's been signs that she was being trained for something important since episode one.
5. Derpy was shown. The real Derpy, but we'll see if she's back for good and this isn't just a finale farewell to her (remember, the writers want her to stay too, but if they aren't allowed to use her they won't. Hopefully she's back, but if not, we'll still see her in fanon.
6. The word "Alicorn" has finally been used for its proper meaning (it was used in Magic Duel, but it wasn't actually clear if it meant what it meant in our world.
7. It's obvious that they are still aware of the two non-preschool targets (bronies, and fans of the old generations - and yes, I know bronies weren't the planned target).
8. Agree with Twilight's change or not - she is obviously the same character, and that is evident already to me. Also, she did look really cute at the end. ^_^.
9. The costumes that the ponies were wearing at the end, all looked absolutely awesome. Anyone else think it was Rarity again?
Negative Points
1. This is more of an opinion thing, but it kind of messes up my idea on what Alicorns are.
2. The episode probably could have been longer, or split into two episodes. As awesome as the episode was, it could have been that the penultimate episode was about the cutie marks spell, and it ends with Twilight vanishing, and we get an extended episode about her final trials before she becomes an alicorn/princess. I still love how they did it though.
3. I can just see MLP: FIM getting the Harry Potter/Naruto treatment with "alicorn", since no one does they're research these days.
Looks like the positives have it; I'll let this sit here before I explain why even though the Princess Twilight was most likely planned, the Alicorn = Princess thing was a recent choice from the writers.