Luna's just Luna. Sombra was a little underdeveloped for my taste. Sauron is a good comparison. Who cared about Sauron in Lord of the Rings? The real villain was Saruman. He's the guy who actually DID shit. Unfortunately, Sombra had no Saruman and the episode suffered for it.
That aside, I thought the jokes were good. Rarity was particularly funny. I also like that Spike finally gets some recognition (and his own stained glass window). The main thing I take away from this premiere is that book Luna was holding at the end. It's obviously important or they wouldn't have shown it in the last five seconds, but what could it be? There's a theory going around that it could be a book by Starswirl the Bearded, since the cover has stars and a swirl on it.
I hope Sombra isn't gone for good. I mean, he got shattered into a million pieces so I realize it's unlikely he'll come back but I would like to see him live up to his full potential as an antagonist in this universe.