Linux

Okay, so, please forgive me ahead of time for the following rant. To be blunt, you did ask. 😛

• It often doesn't use common UI/UX conventions found in most other editors
• It has no polygon tools.
• The Lasso tool is called "Free Select" instead of, you know, "Lasso" like every other software under the sun calls it. (Though I admit this in itself is merely a nitpick, it is indicative of the larger trends.)
• The text tool is so bad. Honestly, I don't even know how to put how it's bad into words, but just using it is...painful...in comparison to Paint.\NET, Pinta, or even MS Paint back on Windows. Other people can probably word the problems with it better than I can. Sorry I can't be more descriptive.
• It doesn't have Lanczos resampling for resizing images (tbf neither do many others but still Paint.\NET does and so that's a point against it. (If you don't know, Lanczos is visibly superior in maintaining fidelity when downscaling an image, compared to linear, bilinear, cubic, etc.)
• The currently active layer seems to randomly change, so that one minute you're doing something and the next nothing is worked, you wonder "what the hell" and then finally after 10 minutes of searching you find out it's because the layer has changed and now you need to go click on this one obscure option. (I don't remember what it is. Select > Select None maybe? Anyway, I've had it happen where the option doesn't even do anything.) It completely throws my whole game off and I've never once, even once had it happen until I started using GIMP.
• The default UI/UX is very rough around the edges. Just to make it minimally usable for me, I had to install PhotoGIMP over GIMP and spend 20-30 minutes customizing the layout and keyboard shortcuts. Speaking of...
• The default keyboard shortcuts are kinda wacko. For example, Zoom In, Zoom Out, and Fit Image in Window (basically zooming in/out but to see the whole image in your window) is +, -, and Shift+Ctrl+E, respectively; while most other programs have it as ctrl++; (and/or ctrl+=), ctrl+- (and/or ctrl+NumpadMinus), and ctrl+0 (and/or ctrl+NumpadEnter). Also, you cannot use tab or ctrl+tab to move to the next or previous tab, respectively, because tab is a excluded key for keyboard shortcuts. (I think I was once told it has to do with a limitation in GTK, but that's ridiculous as Pinta has been able to do it for years.) There are countless other inane defaults for the keyboard shortcuts as well, frankly.
• You cannot use LMB or RMB to switch between the primary and secondary colors selected. You have to use X.

These are only a few of the most severe frustrations, annoyances, and hair-pulling-out moments for me with regards to GIMP. I'd never have even tried it out if Pinta hadn't made the ass-backwards decision to move to the stupidly minimalistic and less functional GTK4 adwaita UI and if Paint.\NET worked. (I can't remember why it doesn't wanna work; I think it has to do with a dependency. I know it's not the .NET framework since that could be handled by Mono IIRC.)