"What's crazy is Gimp has always felt user-hostile to me."

You're not wrong, I wish you were but you're not. By any measure GIMP is a dog of a program. I wish it weren't so as I stopped upgrading my various Adobe products some years ago, but alas it is. It would take me as long as this 'The Decline of Usability' essay to give an authoritative explanation but I'll attempt to illustrate with a few examples:

1. The way the controls work is awkward, increasing or decreasing say 'saturation' is not as intuitive as it is in Photoshop and the dynamics (increase/decrease etc.) just isn't as smooth as it ought to be. Sliders stick and don't respond immediately which is very distracting when you're trying to watch some attribute in your picture trend one way or other.

2. Most previews are unacceptably slow; they really are a pain to use.

3. The latest versions have the 'Fade' function removed altogether. I use 'Fade' all the time and I don't like being told by some arrogant GIMP programmer that the "function never worked properly in the first place, and anyway you should use the proper/correct XYZ method". You see this type of shitty arrogance from programmers all the time[1].

4. GIMP won't let you set your favourite image format as your default type; you're forced to GIMP's own XCF format and then export your image into say your required .JPG format from there. (I understand the reason for this but there ought to be an option to override it, if the GIMP developers were smart they'd provide options for various times, for instance: 'session only'.)

5. As others have mentioned, there's icon and menu issues, menu items aren't arranged logically or consistently.

Essentially, the GIMP's operational ergonomics are terrible and there's been precious little effort from GIMP's developers to correct it. (GIMP's so tedious to use I still use my ancient copy of Photoshop for most of the work, I only then use the GIMP to do some special function that's not in Photoshop.)

[1] The trouble is most programmers program for themselves—not end users, so they don't see any reason to think like end users do. (I said almost the same thing several days ago in my response to Microsoft's enforcing single spaces between sentences in MS Office https://news.ycombinator.com/item?id=22858129 .) It doesn't seem to matter whether it's commercial software such as Microsoft's Office, or open software such as the GIMP or LibreOffice, etc., they do things their way, not the way users want or are already familiar with.

Commercial software is often tempered by commercial reality (keeping compatibility etc.) but even then that's not always so (take Windows Metro UI or Windows 10 for instance, any reasonable user would have to agree they're first-class stuff-ups). That said, GIMP is about the worst out there.

"At this point it feels like a prank that's been going on for a quarter century."

Right again! GIMP developers seem not only to be hostile towards ordinary users but there's been along-standing bloody mindedness among them that's persisted for decades; effectively it is to not even consider ordinary users within their schema. Nothing says this better than the lack of information about future versions, milestones, etc. All we ever get are vague comments that don't change much from one decade to the next.

Perhaps it would be best for all concerned if GIMP's developers actually embedded this message in its installation program:

"GIMP is our play toy—it's prototype software for our own use and experimenting—it's NOT for normal user use. You may use it as is but please do not expect it to work like other imaging software and do not bother us with feedback for we'll just ignore you. You have been warned".