But I don't know what "zoom" means in the context of a window. And the inconsistencies of my experiences with various apps didn't make it clear either. (e.g. the infamous mini-player for iTunes)

As for Shift-Click I tried it with Chrome and I thought you had made my day. However it didn't do anything different on Messages, nor Safari: it just stretched the window vertically. In my short survey of currently open apps, half were stretching vertically while the other was doing fullscreen. (be it Shift-click or plain click) Only Chrome acts differently.