When you start to try to "reinvent" what's more than fine enough already, that's when you know you're running out of ideas :P
Going from an iPhone 4S to a Nexus 6, one of the biggest things I miss is the home button. Sure, the digital one on the Nexus doesn't break, but it doesn't have a satisfying tactile response either (and takes up space on the screen, but that's another story :S)