The best way to present visual content is to use the entire screen for it.
Imagine you would like to look at a single item, wouldn't the best way to look at it be to blank out every other information on the screen in favour of visual focus onto this one item?
Fullscreen does exactly that.
Gnome Terminal knows Fullscreen mode (F11), Open Office Writer knows F11, Gedit knows F11, Epiphany has rudimentary support, Firefox, , what have you..
Examples for apps that do not comply with this unspoken rule are:
Nautilus
Evolution
Transmission
Contact List
Fortunately, Compiz has "window rules" which can allow for toggling of fullscreen mode for any app.
Unfortunately, this feature is buggy, as gnome-panel will sometimes cover a fullscreen app, instead of disappearing behind it or autohiding.
UNR used Maximus by default already in order to utilize the display screen real estate better..
how would you like a consistent DE with F11 to send any window that is not a yes/no dialog into fullscreen?
Yes for consistency, but I'm not sure how needed this is. I for one would never have a need to have transmission or nautilus fullscreen, or the contact list. These would all be better off with a fixed width and getting maximized vertically.
But that is a different discussion. :P
Consistency would be good. I can't think of any reason why not to use F11 as a system wide fullscreen key, other then many laptops currently shipping use F11's default behavior as pause/play media keys, with F11 hidden behind fn.