Hilarious episode. They're still on a roll with this season. Seeing Cadance again was a lot of fun, and she really got to exhibit more varied personality traits this time around than her previous appearances. While I agree with what Mark
said about Rainbow Dash (how it was odd that she just took off, leaving the rest to deal with Discord), thinking about it, I actually don't have too much of a problem with it. At this point, they've come to... not trust
Discord exactly, but they're fairly confident that his days as an actual MENACE are pretty much over. He's had ample time to show that, and also ample time to be a menace again if he really wanted to (especially after they lost the elements), but he's been content to just be the most annoying practical joker in Equestria. So there's not much chance of actual danger, and it seems like being squished into that group hug with him was just more than she could take.
I also liked the crazy sandworm thing - really interesting critter, hinting once again that outside the realm of Equestria and all of its magical, princess-powered protections exists a dangerous and wondrous fantasy world. Seeing Twilight and Cadance fighting it off was really damn cool, too!