Not sure what to think about this.  I am sure I won't disable the Preferences setting because I am used to working with size and hardness.   Seems to me if you need to hold down a modifier key to toggle between hardness and opacity, then that same finger could just as easily hit the number keys.    Hmmm... just tried it and it doesn't work. you need to let go of the Alt key to change opacity.  My thinking is that it _might_ be useful to see the affect of opacity change on the fly, but that would not work with using a modifier key with Alt right click, because you are looking at the quick mask colour, but then again, hitting a modifier key could change from quick mask colour to foreground colour, and show its opacity.  That I would find useful.

you can change opacity by shortcut:

[Enter]+[UP]+[Enter]+[Esc]

For down use down arrow key.

I use it on cintiq and ituos.

For bigger change repeate [UP]'s as much as you like

Alt + Right-click-drag right and left would change Size (as it is now)

Alt + Right-click-drag up and down would change Hardness (as it is now)

Alt + Control + Right-click-drag up and down would change Opacity. (new)

I would absolutely love this functionality! Perfect solution. And I'm surprised it isn't already added; seems obvious. If added it would really enhance the already smooth Photoshop painting experience. When brushing, I love the new hardness change-on-the-fly thing, but find it jarring and unintuitive having to then hit keyboard numbers to or dragging the Brush Tool opacity scrubby bar to alter opacity.

Has this idea been submitted to Adobe? If not, let's get the PS team to respond and see if we can't get them considering it, officially.