Gimp is just full of little annoyances that interrupt your flow and make it difficult to work efficiently.
Recently I had to make some end-credit cards for a short film - ie. white writing on black backgrounds. Gimp managed to make even that painful. Things like, placing an immovable dialogue over the handles of my text box so I can't get at them. If I try to replace text, it reverts to its arbitrary choice of font / size so I have to type new text in the middle of my existing text, then delete the original. No "duplicate layer" shortcut. Just gratuious annoyances that are good for nobody, and bad for everybody.
Edit: I have a feeling you might be a Gimp dev, in which case I should be more grateful for the work you do, which I still use and benefit from even though the UI frustrates the hell out of me.
Edit2: I just found the Duplicate Layer shortcut. Not sure how I missed it before.
PS allows moving around a few sliders while looking at the screen to see how things change and nail it when the results look visually good. I bet that is critical for most PS users. It is for me. I paid for an (educational) PS license just so I can edit my photos before I print them and hang them on a wall.
One example: doing levels. One slider move back and forth to see which areas are overblown in each channel. One extra keypress to switch between final result and overblown only. That, to me, is so powerful. I can decide on the fly, "well, I lost detail in yellow channel here, but extra detail there is clearly worth it".
I am sure GIMP is great in many ways, but I guess their target user group is different. Their interface just does not work well for me (and I did try), they focus at different things, etc.
I hate Adobe (both their licensing and privacy policies), but CS is just so good. It is one of the three programs for which I still have a Windows partition on my computer (I do most of my work in Linux).