The best new feature of Mountain Lion (it's NOT what you're thinking)
In Snow Leopard, Exposé was wonderful piece of software that would show all of your open windows in a neat view on the desktop. When Lion was introduced last year, Mission Control was created, and it REALLY messed productivity up. Instead of seeing all of your open windows, it would show you a peek of them, bunched together by app. I still haven't truly gotten used to Mission Control. However, with the introduction of Mountain Lion, there is now an option to ungroup windows by application, giving Mission Control a much more Exposé feeling. It isn't Exposé completely, but I can definitively say that this is probably going to be the most appreciated new feature in Mountain Lion for me.