Mine was that there is no split screen multitasking on the iPad Air, but if I think about it that's more a disappointment with the iPad itself - it didn't perform very well out of the box with only 1GB RAM, I can't imagine what it'd be like running two apps at once.
Otherwise, I would've liked a system wide dark mode which intelligently switches on for when I have to Google something/text someone late at night. Would be a bit easier on the eyes.