At work, I tend to have a web browser opened (usually Firefox, with a handful of tabs), two or three gnome-terminals (some of which are tabbed), and four or so emacs windows. I tend to move the windows around all the time. I currently have two 1680x1050 monitors, but I'll probably move to one 2560x1600 (or 1400 or however they come these days) and one smaller one off to the side eventually. I've never used multiple desktops effectively -- I'm usually working roughly on one task at a time, but that task involves looking back and forth between reference code (in emacs), reference material (in Firefox or maybe evince), and terminal output.
This is a lot of windows. I am very unproductive without some kind of efficient navigation. I have decent short-term spatial memory, so finding things on a dock (currently Cinnamon's, but I'm not really partial) is efficient; using fancy overviews is not because windows move around and, zoomed out, they all look more or less the same. (I can imagine this being reversed if I edited graphics or something like that.)
The Alt-Tab to change applications feature is counter-productive for me. My emacs windows are <i>not</i> all related to one task -- I'm much more likely to have an emacs window logically paired with a terminal.
Compositing is slow. This may not be Gnome's fault -- I have a low-bandwidth Radeon, and the drivers are not up to the i915 standard.
The top bar doesn't help me. I want a clock*, Pidgin (so IMs don't pop up but instead get my attention when my eyes wander up there), and a list of running programs. That's it. A name telling me what program has focus does nothing for me. Similarly, the notifications that hang out invisibly on the bottom of the screen are useless, because I can't see them. Programs like Pidgin and Remmina are completely usable on other WMs; I don't see why gnome-shell should make them suck.
On my laptop, this is all different. I'd still like a dock of some sort, but screen real estate is at a premium, and there simply isn't space for the mess of windows I use. gnome-shell does pretty well in that environment. (With caveats -- the focus on pretty black system-modal dialog boxes is bad -- the first time you try to connect to a wireless network and need to look up the password in your handy list of passwords, you'll curse at it, because you can't actually get to your list of passwords. The fact that the text entry widget doesn't really work right is a bug, not a mis-design, but fixing the bug wouldn't actually make the dialog box as a whole work better.)
P.S. I wholeheartedly support the elimination of fallback mode. If people want a different WM, they should use a different WM.
* Can someone please give me a clock that doesn't show seconds on the dock / top-bar but that *does* show seconds when I click it for a full calendar? This has always seemed like the Obviously Correct (tm) way to do it, but nothing works like that. (Again, not a GNOME 3 regression; I don't think I've seen any built-in clock get this right.)