I'm a fairly casual / infrequent user, but they've already addressed most of the things I found infuriating in the past.

The big remaining thing I can think of is the "layer boundary" thing, which (it seems to me) is very confusing for new users and inconvenient for all users.

I also have a vibes-based feeling that when I used to use Photoshop twenty-five years ago I was a lot more productive than I'm able to be in GIMP, but I can't be sure if that's just because I used to do more graphic stuff back then and I've never picked it up to the same degree. I find myself having to hunt for things in menus a lot, but that might be a me problem. Photoshop also took a while to get used to.

Edit: I basically agree with the people complaining about Save and Export being unexpectedly separate, although also it's not that hard to press Ctrl+E instead of Ctrl+S.