I must be very submissive to Apple’s will. I am already used to Yosemite. There are a few UI elements I’m not particularly fond of (e.g. the flat window close/minimize/maximize buttons), but I find myself actually liking the dock better. It’s easier to see which apps are open or not. Apple Mail also looks much nicer, though without plugins it is still harder to use than Gmail for finding and filing things.
It also feels faster than Mavericks.
Anyway, here is a nice article summarizing some interesting new features:
doug@resistance is futile