I left Gnome 3 for Mate early on and I never looked back, but here are the issues I remember.

First, dialogs could not be dragged to the side. It took me less than one day to realize that, when it comes to naming a file, I need to look at the document to remember, for instance, the name of the institution I'm writing to. If I can't move the Save dialog then I have to cancel, find the name, remember it, and start the save process again, at which point I also lost the folder I had navigated to.

Showing app icons without categories is useless to me because sometimes I don't know what the app is called, only what it does. I know I have a webcam app and I know it should be under Video, but I have no idea what it's called. Gnome's flat hierarchy makes it harder for me to find what I'm looking for.

Removing the individual window menus and the icons for minimizing and maximizing kills the muscle memory I've built over the last 30 years - same reason why I hate that the X button on Mac doesn't really close the program.

But on the topic of Mac, that's for me the main problem: that Gnome 3 wanted to be MacOS and copied their ideas regardless of whether they make sense to their users. I know the things I mentioned above may sound small but those are constant papercuts that remind me every 5 minutes that I didn't use to have so many papercuts. I can't imagine that anyone did a proper usability test before dropping what, to me, were breaking changes.

There were other factors like the attitude of "this is how it is now so take it or leave it", but the whole process led me to realize that I don't want to think about my window manager and that, if I'm thinking about it, then it's getting in my way of doing actual work.