None of these features -- I see Mountain Lion as just a package of "bells and whistles" that really brings no groundbreaking new features or functionality to the OS that Lion did not have.
I would like the Operating System to take full advantage of four, six, or twelve cores, and not just one or two. I would like applications to take full advantage of the Retina display on the new MacBook Pro. I would like actual SIRI "full" functionality built into the system, full voice command recognition and navigation, and full AVI/WMV/XVID/DIVX video playback built into QuickTime standard...something Apple has never delivered!!
QuickTime needs to be re-written from the ground up. The only way to play various formats currently is to resort to a 3rd party program like VLC. This is unacceptable in the year 2012.
Apple also needs to support audio formats like FLAC -- currently there is no way to rip or playback FLAC in Mac OS X or iTunes, without using a 3rd party program.
Apple needs to improve their TextEdit program, it is seriously lacking.
These things and many others, very simple improvements to add, but Apple won't do it.