Federico Viticci wrote an excellent review of iOS 12. Stephen Hackett (a.k.a. 512 Pixels) summed up the best of it in a couple of sentences which I decided to repost, for posterity’s sake:

With iOS 12, Apple wants to rectify iOS’ performance woes, proving to their customers that iOS updates should never induce digital regret. Perhaps more notably though, iOS 12 doesn’t have a single consumer feature that encapsulates this release – like Messages might have been for iOS 10 or the iPad for iOS 11. Instead, iOS 12 is a constellation of enhancements revolving around the overarching theme of time. Apple in 2018 needs more time for whatever the next big step of iOS may be; they want iOS users to understand how much time they’re spending on their devices; and they want to help users spend less time managing certain system features. Also, funnily enough, saving time is at the core (and in the very name) of iOS 12’s most exciting new feature: Shortcuts.