Well, that was quite a shocker. After a spell goes awry, we finally learn what Twilight's destiny is. It'll be interesting to see where the character goes from here.
As for the episode itelf, it was pretty good. Seeing five of the mane six with mixed-up cutie marks and destinies was something else, and the songs were just fantastic. I liked all the continuity nods; Looks like they used clips from pretty much every episode. The moment where Twilight transformed was just incredible, and the final scene, with the new princess standing with her peers, was awesome. I can't wait to see what happens in season four.