Threading is still fucked in Mail.app. It was fine in iOS 9 but has been completely borked since 10.
Music.app keeps getting more bloated and harder to use.
Killing off 32 bit apps was a dick move.
Notification Center is still a complete mess.
Control Center isn’t customizable enough.
The send button in Messages.app is still to damn hard to press.
But hey, at least they finally fixed the bug where Calculator.app make those stupid sound effects even with key clicks turned off.