Having fullscreen mode working on several displays has several problems, mainly in switching them. How would you switch them in Mission Control? How would you switch them with mouse gestures.
Currently you see the all the desktops on all the monitors in Mission Control. Clicking on any desktop changes them on all monitors. So you decouple it -- click on a desktop on the "left" monitor and only the left monitor changes desktops.
As far as mouse gestures are concerned. Lion also got rid of that icon in the title bar that allowed choosing a space. Just bring it back. If it's good enough for most Linux distros and Solaris for the past decade+ it should be good enough for OS X.
I think what they really need is an implementation that combines full screen and spaces, meaning that you assign apps to a certain space and when you full screen them they are full screen in that space but don't show as separate full screen apps.
Which is what we had in Snow Leopard!
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The bottom line is that Apple doesn't really care about multiple monitor support. Having run multi-monitor Windows for about 10 years and OS X for 5, OS X has always failed in comparison.