Original Link: http://www.apple.com/osx/preview/design/

From Apple’s Yosemite design page:

Take the red, yellow, and green “stoplights” in the corner of every app window. Not only have we streamlined their look, but we’ve also updated their functions. Close, minimize, and maximize are now close, minimize, and full screen, eliminating the extra full-screen control and consolidating the window controls in one place.

I use full screen mode on several apps through out the day, I actually like the distraction free option it provides, so for me, this change makes sense. It’s what most people probably expect to happen.