Zion, named after my favorite national park, is an icon set designed to mesh with OS X Yosemite’s refined visual language. In Yosemite, toolbars have been simplified, favoring solid-colored circles for close, minimize, and maximize; windows and content feel lighter; and many frivolous elements like drop shadows and gloss have been scrapped.
However, many of Apple’s own icons fail to reflect these system-wide UI changes, instead clinging to the realistic renderings of apps’ physical counterparts. Zion is meant to address these omissions, instill a system-wide consistency, and, most importantly, make our docks, folders, and desktops look a little more friendly.
UPDATE: El Capitan has hamstrung Zion by making the OS’ core system folders uneditable. You can bypass this by disabling some stuff via terminal. 3rd party app icons, such as Adobe products, Twitter, and the like remain unaffected, but without disabling System Integrity Protection via Terminal, you will be unable to swap things like system folders and the finder icon. :(