- Supports only 8 bits. My scanner has 16 bits/channel. I have been using cinepaint, which now is going through a transition period, but Photoshop is definitely nicer than cinepaint for photo editing.
- No support for color profiles. This is a killer if you want to do any kind of digital darkroom with some accuracy.
- No decent support for stitching photos to make panoramics. Before you say that you can twiddle with layers to do this, go see how Photoshop handles this, there is a huge difference. Photoshop can detect similar areas and distorts the photos (to make up for perspective change and lens distortion) to stitch them together properly. In GIMP it's hopeless.
I use linux for everything else, but for photo editing, Photoshop IS much better. Also, the GIMP code is an undocumented mess. At some point in time, I wanted to hack into it to add some functionality, and I spent 2-3 hours staring at the code without being able to figure out how to access the image pixels. At that point, open or closed source, what's the difference to me?