Apple explains one of the changes they are making to the OS in Yosemite thusly:
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 think I’m going to miss the “maximize” button. I say that parenthetically because it isn’t really a maximize button. To be more accurate, it is sometimes more than a minimize button, but it isn’t consistent. On a browser window, for instance, it is a “fit to content” button. If your window is open wider or taller than the content it contains, the button will shrink the window to a point that the content fits perfectly inside it. Finder windows behave the same way. Most application windows, on the other hand, will just enlarge to fill the screen. A second click returns the window to its original size.
In Yosemite, the button will put the application into full screen mode, which is entirely different. The ability to fit to content or fill a screen with one click is just going away.