They're pretty much identical...
I beg to differ.
old icon: new icon:
bevel around perimeter no bevel around perimeter
square corners rounded corners
eyes squared at top and bottom eyes rounded at top and bottom
rectangular ends of smile rounded ends of smile
glossy color gradient satiny color gradient
gradient changing abruptly at eyes gradient consistent throughout
duller blue color brighter blue color
The actual Yosemite Finder icon differs slightly from the one linked to by Dianne: the vertical line delineating the nose and/or the boundary between two faces is of a much lighter weight than the smile, which makes the icon a look a little goofy at small scale, as in the Dock (at least on a non-Retina display).
On the whole, though, I'd have to say I prefer the newer icon myself.
