The kind of computer usage I think Gnome 3 is targeting
Back in a comment on Gnome3Out I mentioned feeling that the Gnome 3 developers are targeting people who use their computer in a significantly different way than I do. Because I've been using Gnome 3 again recently, I feel like trying to explain that today.
While Gnome 3 has a number of interface differences from Gnome 2 and other desktop environments, two of them stand out very prominently (at least to me). First, a great many interface elements are built around the assumption that you only want one window for any particular program. Working with multiple instances of the terminal or Firefox is awkward for reasons well beyond that you have to shift-click the appropriate icon to start the second one.
(For instance, once you have all instances minimized clicking on the app's icon will only un-minimize one window; clicking on the app's icon again will re-minimize that window instead of un-minimizing the next one.)
Second, Gnome 3 fairly strongly wants you to keep all of the windows you're using on the screen (in some workspace). Minimization is not all that accessible and then Gnome 3 works quite hard to conceal minimized windows. In Gnome 2, minimization is accessible as a button and by clicking on a window in the taskbar, and then minimized windows are accessible in the taskbar. In Gnome 3 there is no taskbar; minimized windows are mostly inaccessible (especially if you have more than one window for an application) until you reveal all windows and then sort out which ones you want. The net effect is that it is significantly more work to flip back and forth between a set of windows in Gnome 3, especially if you have several windows from the same application.
As anyone who's looked at my desktop can tell, my style of working uses a lot of windows from the same applications (prominently terminals and browser windows), many of them minimized until I need them again. The same holds true on my laptop, where I do less but I have less screen real estate to do it in so I'm constantly shuffling various windows in and out of visibility. This style is a terrible match for Gnome 3, which really doesn't want me to work that way. To stereotype a bit, my working style is multi-tasking while Gnome 3 is a quite single-tasking environment. My view is that Gnome 3 is built for people who run one thing at a time, or at most no more things at once than all fit on their screen or screens (and they quit out of applications when they switch from one to the other).
From a certain perspective this makes user interface sense. Managing the hidden state of minimized windows is a cognitive burden and things like the taskbar are visual clutter (if you don't have minimized windows). People who don't really multitask are probably decently well served by the Gnome 3 interface, especially if they don't switch back and forth between applications very often. The single-window application unification helps by avoiding confusion if people forget that they already have a copy of the application running but minimized and we've already posited that these people multi-task and multi-window only rarely.
(I know people like this; they already run only one copy of every major application that they use, often without minimizing them.)
(I'm aware that I may be ascribing more coherence and planning to the Gnome 3 design process than it actually had. Allow me my potential charming illusions.)