IMO, look and feel is hardly the biggest failing of the GNOME system. There are more fundamental problems with their user philosophy. Years ago when a new set of workstations were deployed where I work everyone had the option of running either GNOME 2 or KDE 3. Officially the admins only wanted to support GNOME, but within a short time everyone in our location was on KDE 3.
Why? Well it turns out the admins never really did a thorough test of our tool flow on GNOME. We use a lot of expensive tools that come from legacy Unix backgrounds (they aren't recent GTK devel), so it turns out we had major problems with things like focus stealing. This would be where the app would pop up a messagebox and GNOME would happily yank you from whatever desktop you were working on to wherever the messagebox was. At the time there were no options in GNOME to handle this kind of thing, whereas KDE had a number of focus stealing controls.
Then there was the issue of resizing windows. At the time GNOME had one method of resizing windows, and that was to continually redraw the content in it - no wireframe or outline methods, only continuous redraw. That's great and all if your most complex app is a web browser, but when you got an app showing a couple gigs of visual data and every window resize event triggers a redraw, it quickly locks up the machine.
And then there was the question of the right-click menu. WTF was with this menu. It was loaded with a bunch of useless options for creating folders and crap. It was like someone who had never used a Unix machine before just decided to shoehorn in some crap there so the menu did something. KDE at least allowed the menu to be customized into something useful.
This is all regarding GNOME 2 at the time, but it gets to the core of what I perceive as GNOMEs problem - and as I understand it, this is both widely understood, and truly a development target of GNOME (and I fully expect GNOME 3 to be no different) - and that is that the GUI is not designed to be flexible or changeable, it is designed to be rigid and idiotproof. They are providing a fixed GUI interface for the lowest common denominator of user, and anyone who wants something different can STFU.
This is of course further compounded by their method of burying the GUI settings in a hundred different files across a dozen hidden directories, perhaps wrapping it in some obscure XML pseudo-code, so nobody can figure out WTF the options really are or what they do (perhaps it's some kind of subtle method of eliminating those annoying hacker types who might undo their GUI "vision"). KDE is no better in this regard. I remember when at least one GUI I used to use kept its menus in plain text format that was easily understood and modifiable, what the heck ever happened to that concept?
I'm sure if I were to relate to a GNOME dev the problems I had with focus stealing, he would turn around and tell me the problem was with my app, not the GUI. And if I were to relate how I like to launch programs from the right-click menu I would be told I'm doing it wrong and I should learn how to do it the "right" or "better" way. And thus I become yet another alienated user who has moved on to something else. Radically changing an interface and then pushing it as a rigid right-and-only way is going to piss off a lot of people. Lots of people left KDE when they did it, and the same will happen to GNOME.