Camera shutter buttons have a two-stop action. Half-press them to lock focus and aperture settings, fully press them to take the picture. There’s a delightful tactile indent at the half-way mark so that your fingers know what’s going on. Let’s borrow this two-stop action for the home button. Press half-way to go to the app’s main screen, all the way to go to the phone’s main screen. If you need to fully escape mash the button. If you just want to head back to the main-screen of the app, tap lightly.
I agree that the need is real, but there is already a superior solution to this problem: long presses.
If you agree that half presses are a good idea, surely long presses are an even better idea, because in a way, it’s similar to leveraging Fitts’s Law, but for buttons.
Fitts’s Law states that the bigger the target is, the easier it is for users to reach it. Mac’s menu bars are very easy to reach because their vertical size is virtually infinite: just move your mouse all the way to the top, it will bump against the screen and it will be positioned on the menu bar.
I’ll argue that the same applies to long presses. Instead of encouraging a design where you only half press the button (very small target area), press the button all the way (infinite target). Then either release right away or keep the button pressed for a short while. The half click works well on cameras because of the way you hold it, but I have a feeling that having similar accuracy on a phone will prove to be more challenging.
Strangely enough, Gruber likes Aza Raskin’s idea, which, in some way, acknowledges that Android’s long presses are not such a bad idea after all (but of course, he would never admit that).