What happened to Katy? For as much shit I've given her over the years, she's usually been good at crafting hooks... but all the choruses here (except Roar and Birthday, it seems) feel so tuneless. They're mostly either a repeated phrase or word (her delivery of Unconditionally is particularly awful and I will hate every moment of its chart reign) or they start out promising but fizzle out unceremoniously (Dark Horse, Double Rainbow... although DR's chorus will obviously be expanded on later in the song). You had ONE job, Katy.
I appreciate the Big Hoops shoutout, tho.