A long time ago, Anki used words like ‘forgot’. Some users felt it had negative connotations and they said they felt bad when pressing it, so the name was changed to something more neutral.
Regarding putting space between again and the other buttons: I imagine the asymmetry will trigger some users’ OCD, and it feels a bit wasteful to permanently increase the minimum window width for the sake for demonstrating the difference.
The buttons are documented fairly early in the manual, but it is true that their differences aren’t explained well inside the UI. As I’ve mentioned before, at the least, we could improve the tooltips shown when hovering over them, though some users don’t notice them.
I also have no objections to colors (though I think there should be more than just red/green), and the mobile clients already use them - the main issue with introducing them to the desktop is that some users will prefer what they had before, which will mean yet another option. The bottom bar is also likely to be reworked at one point when the reviewer and top/bottom bars are merged into a single page, so it would be more efficient to hold off on that work for now.
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.