I can't tell you how many times I've heard Gnome or KDE insulted by some militant user of a competing GUI, usually in the framework of "usability is pretty terrible."
It's not that "usability" as a concept is already nebulous to begin with, but that it is presented as this objective score that one can actively compare to other scores. Usability means something different to everyone. K Mandla once said that there is no such thing as a bad interface. I have to agree with him here.
I mean, just look at the word. "Usability" means, the ability to be used, or in our example militant's use, how easy it is to get things done. The fundamental flaw in thinking that usability can be understood objectively is that assumes everyone uses a computer in the exact same way, which we all know is not true. Furthermore, this also assumes that even a single person will use a computer in the same way they always do it, 100% of the time, for the rest of their lives, which is also false.
I'll use myself as an example. My computer needs change daily, or at the very least, weekly. Sometimes I need a web browser and nothing else. Sometimes I'm doing heavy photo editing. Sometimes I'm writing code or a piece of fiction. Sometimes I'm reorganizing files, deleting cruft and reordering the rest. And sometimes I'm not interested in using the computer at all, and just want to check my e-mail.
Naturally, these are very minor, tiny examples of just a week of computer using in my life. But even these few things need drastically different programs and interfaces to work just right, and even then there's always room for improvement.
But who is to say that Firefox is better than Chrome for my use? One and only one: Me. Who is to say that Gnome is better than Windows XP for managing my files? Me. These are subjective, imperfect, vague and utterly personal judgements to make, and ones that cannot be forced on to others easily, if at all.