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:
Maximize the window to fills the visible content: Option + Click on green button
Maximize the window both in width and height to the current desktop (like Google Chrome): Option + Shift + Click on green button
Double click the title bar will now maximize and de-maximize the window.
Now you will be able to maximize your applications again :)