There are a couple of constants that i find disturbing.
love.joystick_axis_horizontal and similar. Given that there is a love.joystick.getAxis(...) function, where do you use these constants?
My concern is mostly that joysticks can have more then two axis, and therefore this system with constants representing each axis is not very flexible. I propose removing these constants.
I have a similar concern with the mouse buttons. If a mouse has more then 3 buttons, it would be nice to be able to access these buttons. While on the mouse subject, would it be an idea to integrate Many Mouse for multiple-mouse games? Not high priority i know
I've considered Many Mouse, but I haven't really looked at it properly. As you say, it does not have a very high priority. Before version 1.0, though, we should consider adding this, and possibly review the entire input system too.