The differences are many and varied, starting with memory usage. An LXDE installation is much more frugal with ram than Gnome. Mint Gnome 32 bit typically, on my systems, uses about 220-250 MB of ram just to keep the desktop displayed. LXDE uses about 100 MB or less, depending on the Wallpaper in use and what background daemons you have running.
The standard LXDE panel is also completely different than the Gnome panel. It may look similar on first inspection, but it is configured entirely differently and, in some ways, is less flexible than the Gnome panel. Of course, in other ways, the LXDE panel is superior to Gnome. Try it out and see for yourself.
Gnome uses Nautilus as file manager, LXDE uses PCmanFM. Totally different.
There are many more differences, too many for me to take time to list. Try Mint 11 LXDE for a few days and you will see and understand.
By the way, installing the LXDE desktop on top of Gnome will not give the same result as running a version designed for LXDE as primary desktop environment.
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