I like the mac like global-menu or appmenu whatever you call it. But I was wondering if there is a way that I use it only when I maximize the application window ?
I'm afraid that's not possible right now.
Trusty Tahr 14.04 brings a solution to this. Go to "System parameters"/"Appearance"/"Behavior" than switch "Show the menus for a window" to "In the window's title bar".
The menus will then appear when hovering a non-maximized window's title bar.
I would like to second the request. I'm hoping it will be implemented as an option soon.
In the mean time, removing 'indicator-appmenu' with 'sudo apt-get remove indicator-appmenu' will stop the menu moving to the top bar on all windows. If you have your focus to follow your mouse as I do, this is an essential change (as otherwise you can't reach your menu options). In fact, it's a pretty nasty bug for non-click focus users.
IMO, it would be best for there to be an option to only put the menu in the top bar when maximised, as it makes the most sense (and best use of display space) then.