For me, it has the worst ergonomics of any desktop I've used.
Since I asked somebody else to be specific, I feel that I should be specific.
The horizontal panel across the top of the screen is an egregious waste of space, especially on modern widescreen displays where vertical pixels are valuable.
A graphical task switcher was one of the most substantial UI improvements in Windows 95. GNOME banishers this to usually hidden, minor feature. That's bad design. GNOME is actively hostile to beginners and less proficient users.
In the meantime, its fans claim that it is very keyboard centric, but it does not respect the existing keyboard controls established by Windows and indeed GNOME 2. I was already keyboard power user, but GNOME 3 breaks my workflow for no good reason.
Keyboard centric design is good, but you need to respect existing keyboard controls and existing user interface conventions before attempting to improve upon them. It seems to me that the designers of the GNOME 3 desktop had a very little knowledge of the existing keyboard controls, and simply invented a new set without understanding the existing tooling. Given the extremely fondness and respect for legacy editors such as VI and Emacs, which have heavily keyboard centric design which has not been changed in 40 years so as not to inconvenience skilled users, the same principles should apply to a graphical desktop.
People looking for an editor are told to just learn these 1970s user interfaces. Well, people designing a desktop should just learn those 1990s user interfaces.
The removal of true title bars is a significant impediment to usability. I am using a WIMP desktop and I want to be able to minimise, maximise, roll up, and send windows the back of the stack; none of these possible with GNOME.
I read and write in English language, using the Latin alphabet. This reads from top to bottom of page. Therefore after I have read the contents of a dialog box, that is when I make a decision what to do. Therefore after the contents of the dialogue box is the correct place for the action buttons. Putting them in the fake title bar at the top, before I decide what to do, is a reversal: it is putting the cart before horse.
I do not normally store files on my desktop, except the temporary holding place, but I do use it as a convenient place to store convenient shortcuts to my normal working folders. It's my computer, and therefore I get to choose where to store things. It is not the position of the desktop designers to decide that I cannot store things on the desktop.
One of the major innovations of user interface design in the mid 1990s was a hierarchical view of my folders in the file manager. GNOME removed this. To remove important working features is bad design. To decide on behalf of users which features are important or not and remove some them without soliciting feedback is bad design for an allegedly community centred project.
While having global system search is a useful facility, defaulting to searching for existing files inside the Save dialogue is a deeply stupid move which inconveniences me every single time that I use a GNOME based application.
Intentionally removing features from the underlying GTK toolkit, and making substantial changes which substantially inconvenience third-party apps using that toolkit, is a community hostile move. This is the sort of behaviour that I expect from proprietary software vendors, not allegedly open source projects.
Those are just a few of the first irritations that come to mind in what I consider to be an extremely user hostile design.