Yosemite looks like crap to me. The flat design can go Foxtrot-Off for me. If I want my computer to look like a crappy Windows, I wouldn't have drop over 2 grand for it.
funny that people keep saying this - yosemite isn't flat design. it's flatter, but almost everything still has skeoumorphic design. it's just subtler. only real flat elements are in the sidebar icons. and comparing the transparency to windows? that's all apple - harkens back to cheetah which was years before vista (or whatever win version had so much transparency). and i think it's visually classier.
having said all that, i'm not implying everyone has to like it. to me, the look of yosemite is light years ahead of the past few OS X releases. so all in all: SnowLeopard > Yosemite > Mavericks > Lion. and Yosemite would be my number 1 if the tagging system could be changed back to the snow-leopard setup.
protip: holding the option key above most toolbars or menu items before clicking leaves TONS of the snow-leopard UI intact (save-as, expand window, etc) - it's just hidden a little in favor of yosemite's default-view 'simplification.'