Earlier quoted context omitted.
Daily GIMP user here, pretty much the only thing you can't do is print CMYK.
The problem with GIMP isn't its featureset, but its interface. It's so foreign to anyone who works with graphics that whatever UI metaphors it uses fail to resonate with people, so they just end up confused. What GIMP probably needs is an overhaul that copies what Photoshop does. People understand Photoshop, and it's the industry standard.
Of course, we're offering zero evidence that either position is true, just bald assed assertions. That I vastly prefer multi-window is my only evidence.
>People understand Photoshop, and it's the industry standard.
They've been trained to. It's as intuitive to you as the way home from work. It's as intuitive to me as the way to your house from your job.