It has been a very long time since I tried GIMP (>15 years) to remember everything I found wanting, but as I recall, GIMP lacks both macros and batch editing, the former letting you record a set of actions to a hotkey so you don't have to repeat them yourself all the time, and the latter letting you apply a set of actions to hundreds or thousands of images at once. I would literally have to spend hundreds of hours to do things in GIMP that can be done with no effort in Photoshop, to the point where it would actually be easier to just program something myself from scratch than it would be to use GIMP, if Photoshop didn't exist.
I see that GIMP has since gotten a UI revamp, but the multiple window UI from the time I used it was also unbearably bad and one of the main things that sticks out in my memory.
I see that GIMP has since gotten a UI revamp, but the multiple window UI from the time I used it was also unbearably bad and one of the main things that sticks out in my memory.