Every touch-based one, for starters. I don't know what I can tap, or what will happen if I tap them. Or long-tap, or double-tap, or lightly-tap. Or what the various swipe directions mean, or how swipe-from-edge differs, or the differences between a 1-, 2-, and 3-finger swipe. And nobody ever guessed how to "undo" on iOS without being told.