Alicorn Magic is what was previously called Dark Magic. Neat. Really, that makes Sombra a lot more interesting, giving that he was using the stuff
all the time. A pity they didn't take five minutes or so in the flashback's to conform that little bit.
Nightmare Moon was rather toothy, and had a really nicely animated tranformation. Headcanon: Luna/Nightmare Moon obtained that throatier Kathleen Barr voice over a few centuries of screaming and yelling, hence why she didn't sound any different during the fight itself.
So, in the previous age, the Elements were CGI. Good eye, Twilight, they were different!
Apparently, much like how the Season 3 opener was Spike's time of glory, the Season 4 opener is Applejacks.
Also, Discord was really well used, especially given that he was technically the villain of the whole episode but really wasn't. I laughed at him throwing seeds at Celestia, mostly because I thought they were, like, chocolate covered coffee beans or something.