Lack of customization for one, either I do things the GNOME way or the highway. Screw that, if I wanted that I would be using MacOS and/or iOS instead since Apple does that far better.
Form factor dissonance for another. GNOME clearly targets the mobile form factor, and it fails me for all the reasons Metro in Windows 8 failed me because guess what: I'm using a desktop/laptop, not a tablet/phone.
Yeah if you value customizability at all, you should probably be using kde. I value simplicity and consistency.
I had issues with ubuntu's unity back in the day and I switched over to i3wm, but I didn't find I used tiling enough to make it worth losing the usability of a desktop environment
I think it's more fair to say all form factors are treated equally, to the possible detriment of focusing exclusively on desktop. I think Gnome does well and is really versatile no matter which form factor you use, and I didn't have much issue moving from Gnome 2 to 3, or Windows to Gnome, or OSX (at the time) to Gnome (I've gone back and forth a lot over the years).
For me, workspaces (which Windows lacked natively until very recently) and Alt+Tab/` are how I get around.
The customizability argument is a solid reason to dislike Gnome, but not for all time. Things do get better each release. Well, except for extensions, which always break.
About configurability, I installed more than a dozen shell extensions and my Gnome desktop looks like and behaves like what a desktop should be for me, quite distant from the ideas of Gnome's developers.
Form factor dissonance for another. GNOME clearly targets the mobile form factor, and it fails me for all the reasons Metro in Windows 8 failed me because guess what: I'm using a desktop/laptop, not a tablet/phone.