Assuming you have a trackpad, why not use the gestures? CTRL + [ is also back in Safari (and other OS X apps).
Not sure why you feel backspace should be back, just because it has the word back in it. It's a backspace key.
Because every browser does it. If you could map it to ONE key I'd be satisfied.
I have the two-finger-swipe, but it takes longer and only works well with the trackpad (and not with the magic mouse).