For me, the major shortcoming is adjustment layers. In Photoshop, you can apply a non-destructive layer/filter over your image to modify parameters such as brightness, contrast, colour levels, etc. You can then directly edit your image "below" this filter, e.g. cropping it. You can then modify the adjustment layer later.
In GIMP, once you modify brightness or contrast, that's it. You can't come back and remove/change these setting later. This has been a requested feature for at least 14 years.
I wanted to quote this, not just for agreement, but to point out that ... yes.. this is a seriously large issue for professional work. I'm not sure that the 'why' of its importance is widely understood around here either so I just wanted to add some detail to it.
I think that the common mindset might be that once you've made a change, you're done, there's very little journey for you after that. That's true in many cases, such as simple photo editing etc. The value in having what amounts to variables in your stack of layers may not seem high enough to warrant Adobe's price.
When you consider that professional work being done means there's an economic advantage to getting done faster, then the idea of being able to create non-destructive templates in Photoshop means $$$ becomes a little clearer. Some time I invest in creating image 1 could mean I spend half the time creating image 2. It also means that if an image is kicked back to me for revision I can really quickly make that adjustment as opposed to re-tracing a number of steps. Again, time is money.
If I were asked to come up with a programming metaphor I'd give you this really shitty one: Imagine people urging you to switch to a clone of Python that doesn't let you create your own modules. Many of them don't need or want to create their own modules, but for plenty of people who have dug deep into it they feel they'd need them from day one, suffering greatly from the lack of that feature.
I've mentioned before that GIMP may be free, but that it wouldn't actually save me money over Photoshop, this is precisely why.