How does maximizing/full screen work in Mavericks and Yosemite? OSX features full-screen mode in recent versions but that's not really the same thing as automatically resizing a window to use the full desktop like on Windows...

And, what is the + button supposed to do exactly? It generally seems to make the current window a bit bigger but not in an obviously consistent way.

Somehow I can't figure out how this is supposed to work (or I'm too used to the Windows way of things), so any explanation would be apprecated.
The green button used to be just for "zooming" the window (at least that's what they call it in austrian systems) - it's what it does when it shows the "+"

If you use that function what it does is it makes the window just so big, that no scroll bars are needed (with the limitation that it can't make it bigger than your screen of course). 
That is the intended behavior, some 3rd party apps might not implement that correctly however.

In 10.10 Yosemite the green button is now used for switching to fullscreen.
It will behave as in 10.9 if you hold the alt-key while pressing it (you will also see the + sign appear).