Apple says iOS 9 will had an extra hour of battery to most iPhones. Certainly battery management is an area where iOS shines. And I say that as an Android programmer.
They seem to be accomplishing that in the most boring possible way: Turn off background updates of things when the battery falls below a certain level.
Interestingly they weren't smart enough to turn those things back on when you plug the phone into power. Only once the battery comes back up.
It's hard coded to the battery level, which... Is pretty dumb from a user interface standpoint.
I probably want to know the things I missed while I was low on battery, once I'm plugged into a power source. And the appropriate time to do that, would be when I plug back in, not 20 minutes later when the battery finally charges back up.
You can really see the Apple UX-focus disappearing before your eyes, under Cook.
It's not like plugging into a power source triggering an event is without precedent in iOS. Your phone will happily start an iTunes WiFi sync the moment you plugged it in, if it finds your iTunes machine on the same LAN subnet via their discovery mechanisms.
This type of thing usually is the result of "ticket based feature changes/big fixes". I'm sure there's a nice ticket somewhere inside Apple marked "completed" that says literally, "stop background updates to save battery when battery falls below 20%" and some software engineer who's quite proud of "accomplishing" it... without any thought to how he or she would do it if they were turning the background sync limiter on and off to save their own battery... because if you're doing it by hand, you'd turn it off when you plugged in, and check the battery level again when you unplugged.