Thinking about on-screen buttons, and having used my mom's Lumia 630, I've realized that I could get behind on-screen buttons with one condition: integration with apps. In other words, say I'm using FB Messenger (since Facebook removed any phone OS' ability to use its own messenger client to use FB messaging, thanks). it has its own bar of buttons at the bottom.
I don't think I'll accept requiring two bars of buttons on the screen. It's bad enough I'm losing real-estate as it is (and the idea that the bottom bezel will be significantly smaller isn't actually proven--it's going to still have a microphone port, right?). Require major app produces merge the bottom bar with the "virtual buttons", its not like they'll be short of space (any app that has 6 or more buttons on the bottom toolbar has issues of its own).
So, in the case of Messenger, you'll have a toolbar that has the add contact button, the refresh button, the search button, the back button, and the home/windows button, and the phone search button. Maybe hide the ones that would be redundant.
I don't want two bars (which, let's face it, are likely not to match for color or design reasons).
At least this way, we're getting some advantage out of the virtual button bar. Otherwise, I'm quite content to stick with my Lumia 830--the three buttons work fine (and it's not as though some aspect of virtual buttons couldn't break), and the light doesn't bother me since I'm not staring into my phone while I sleep (doing so would probably rule out glance too, which is awesome). That's just my position anyway.
And, of course, they need to have vibration or some sort of tactile response. That's a given.