One of the downsides to splitting this discussion between different threads (and not accumulating consensus as it goes) is that we lose even the uncontroversial points that folks have already agreed on. For instance – from a year-and-a-half ago …
Add visual cues to make it clear that Hard is not a failing grade - #15 by dae
Another option would be to add in AnkiMobile’s feedback ticks: a briefly-flashing red X when again is pressed, and a yellow/green/blue tick when the other buttons are pressed. I think that’s a fairly unobtrusive way of demonstrating how again is different to the others.