Unfortunately, there’s no defaults
setting to reverse the functionality of the green button, so you’re out of luck. That said, there are a few things you can do to work around this:
In most apps, double-clicking the window’s titlebar (or, in Yosmite, the combined tool/titlebar) will zoom. Not every app supports this, but in my testing this morning, most did. Once zoomed, double-clicking the tool/titlebar again will un-zoom.
You can set your own keyboard shortcut for the Zoom menu item globally, and any app that supports it will use the shortcut. Open System Preferences, navigate to Keyboard → Shortcuts. Select “App Shortcuts” from the source list, and then click the add button. Keep “Application” set to “All Applications”, for “Menu Title” enter “Zoom” (no quotes) and then set your preferred global keyboard shortcut. I’d recommend ⌘⇧M since it’s the inverse of the standard Minimize shortcut, but it’s up to you.
Install a tool like Moom. Moom adds a popover to the green button which allows you to quickly resize the window, zoom it, move it to a quadrant, et cetera. Moom supports keyboard shortcuts, storing custom layouts and more. You can read more about Moom here on Useful Mac.
There are a few potential answers to this question.
OS X offers a system-wide setting that allows you to control various parts of the interface using only your keyboard. First, you need to change a keyboard setting. Open System Preferences, and navigate to Keyboard → Shortcuts (this navigation might differ slightly on pre-Yosemite builds of OS X, try searching if you can’t find it). You want to change the “Full Keyboard Access” setting to “All controls”:
Now you can use ⇥ to navigate various UI throughout OS X. Next time you’re in a modal dialog, try using ⇥ and ⇧⇥ to navigate back and forth through available buttons. You’ll see a halo around the selected button, and you can use the spacebar to activate it.
Next time you have a modal on screen that has a cancel button, try just hitting ⎋. Most modals support escaping.
Sometimes when closing a document or app you’re asked if you want to save what you were working on. In a case like this, you might not want to cancel but instead choose “Don’t Save”. Next time this happens, save time and use the default shortcut for this in OS X: ⌘⌫