I last used GIMP in 2010 so a few of these might be available now. Anyway here's my list:
- No support for raw images.
- No 16 Bit and 32 Bit color mode.
- No adjustment layers.
- No support for Photoshop plugins.
- Lack of LAB and CMYK color modes.
- No 3D support.
- No video support.
- Integration with other Adobe tools (e.g: copy paths directly from Illustrator).
- Lack of usable transformation tools.
Also even though GIMP supports psd files, once you exceed a few hundred megabytes, it gets really slow compared to PS on the same machine.
The only thing that's better in GIMP is the lasso tool. I hate it in PS, but it's actually usable in GIMP.
If you factor out price (which you can and should if this is the tool that makes you money) GIMP doesn't even come close.