OS X 10.8 Mountain Lion is pretty nice, however I don’t think it really has a killer feature (though John Siracusa’s exhaustive review lists pretty much every change). Notification center is in my opinion the closest to it, and not for notifications itself, but because it can serve as a sort of dash board for calendar making it really accessible. I think that will change my workflow a bit. Lots of subtle UI polish which is nice and appreciated. For those who use Calendar’s location field for phone numbers it’s nice that it’s always completely visible now and never cut off. Apple’s been resolving this slowly over time. OS X 10.6 was terrible when it came to this.
I still don’t understand why my desktop picture reverts every OS upgrade. I’m also a little disappointed HFS+ hasn’t been replaced with something more modern.
Overall a smooth upgrade.