"Full Screen Mode" is not really about full screen. It is an attempt to simplify the concept of virtual desktops, making it more intuitable to ordinary humans, while also putting applications or tasks into a linear arrangement so that they may be navigated more easily with multitouch gestures (task switching).
I have Lion (dev preview) and it's very useful, but for browsers there were always ways to go fullscreen (Firefox, Chrome, etc), even my Aquamacs on Snow Leopard. Having it for everyone is even better.
OSX Lion's fullscreen mode emphatically "is" unique. I haven't seen any other desktop OS that has a system-wide fullscreen (not maximize) button and nobody else is even in the game when it comes to bringing multitouch gestures to the desktop. The gestures are important - the entire point of Lion's fullscreen idiom is to bring the intuitiveness and immersion of post-pc apps to the desktop.
