The huge upside, of course, from my perspective is the improved security model. Many improvements here including ASLR and sandboxes.

El Reg has a quick summary here: http://www.reghardware.com/2011/07/21/mac_os_x_lion_security/
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Rory
MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
MkII, 240Gb in Mark Lord dock
MkII, 80Gb SSD in dock