OS X Mavericks (10.9)
I've had Mavericks installed on our Mac mini for several days and I have to say that the memory compression feature is pretty fantastic. At least I believe that is the reason that the mini is much faster now. It use to be that my kids playing Minecraft with (or without) lots of mods would end up slowing everything down afterward. That doesn't happen any more. A big win, especially for free!
I was somewhat surprised to still see the dock shelf (I mean, that it looks like a shelf) - I thought for sure they'd be getting rid of that, to match the iOS 7 style.
I still need to upgrade our Macbook Air, but it was already fast due to the SSD, so I'm not expecting such a big difference in performance. Or at least not to notice it as much.