Yosemite changed the behavior of maximize button (the green button at the upper-left corner) now, instead toggle between the maximized size of the window and whatever user-defined size was currently configured, clicking this button will now take you into full-screen mode.

Ok Apple, but now how can i have the old behavior ?

Well you have some alternatives:

1. Maximize the window to fills the visible content: Option + Click on green button

2. Maximize the window both in width and height to the current desktop (like Google Chrome): Option + Shift + Click on green button

3. Double click the title bar will now maximize and de-maximize the window.

Now you will be able to maximize your applications again :)

[the Mac Observer]: http://www.macobserver.com/tmo/article/yosemite-maximizing-windows-the-green-stoplight-button?utm_source=macobserver&utm_medium=rss&utm_campaign=rss_everything "Yosemite: Maximizing Windows & the Green “Stoplight” Button"

[Apple StackExchange]: http://apple.stackexchange.com/questions/139884/make-the-green-full-screen-window-icon-on-yosemite-maximize-windows "Make the green full screen window icon on Yosemite maximize windows"