I was actually really surprised with this episode, the premise sounded pretty bad (25 was sounding to be a generic Worf Effect
episode, and 26 sounded like something out of bad fanfiction), but the execution surprised me a bit. They took the character arcs in directions that I wanted to see, but didn't expect (Twilight not really feeling like her role as a princess is important, Discord questioning his friendship with the Mane 6 because it contradicts his nature). I also liked the villain, stealing magic is a neat villain power (with the plus of confirming some neat tidbits about flight and Earth Pony strength being magic). And I liked how this season ended a bit less ambiguously than Season 3, where Twilight turning into a Princess caused a lot of confusing, a new castle sprouting in Ponyville does not, we know that this changes pretty much nothing except for where Twilight lives (and 50 bits says that the throne room is their new "situation room")
What I didn't like was that they sort of handwaved the ending to the "Discord betrays Equestria" plotline by having Tirek betray Discord, and he instantly just went back to being friends with them without really tackling the issue that caused his betrayal in the first place (maybe in Season 5, they could deal with this again in another episode, though, where Discord would look back and realize that maybe Tirek was right and starts to revert to his old ways). Also, I don't really like that we're back to using random Deus Ex Machina artifacts to defeat villains, it's a real cop out for writing a more satisfying ending (although they didn't really describe how Rainbow Power works, so maybe there could be limitations or weaknesses introduced in later episodes).
But overall, good episode, and a good way to close out an otherwise fairly dull season.