GNOME Shell looks like a serious step backwards. I hate to say "as usual", but while GNOME 1.x was quite usable, it's gone downhill from there.
The notion of one active application at a time that's on top is completely wrong, at least for me. Call it what you will, but I've always preferred the freewheeling focus strictly follows mouse, no autoraise, no click to raise style. I often have a big jumble of windows on my screen, and if I get too annoyed I shuffle some of them off to other virtual desktops, but it's how I like to work. I have a big screen (WUXGA), and I'd prefer to move in the direction of a bigger screen (QXGA or whatnot).
I remember all too well using Pagemaker on Windows 3.11. Copying anything between documents was really unpleasant. It would be that much worse if only one window at a time were maximized. Sometimes it's really important to have multiple things at once visible -- I'm working on a Python script and need to see on the fly how changes to the script affect behavior.