A bugbear: iOS' default mail client having the "Mark as Read" and "Move to Junk" actions on an all-selection right next to each other (with no undo option) is maddening.
Oh, wow, I'm surprised that I keep getting surprised at how undiscoverable ios functionality is. I still can't get over being flabbergasted about a decade ago at how the only way to switch their calculator app to scientific mode is by rotating the phone to landscape mode.