As someone who embraces copyleft and is a strong supporter of free software GIMP isn't a replacement for photoshop yet. The roadmap for GIMP should make it possible after 2.10 or 3.0
Here's what I need to switch:
Unified transform tool so I can scale, rotate, shear, etc all with one tool (currently available in 2.9 dev)
On Canvas Preview so I can see what something looks like without having to commit and undo a change over and over again.
16/32 Color Bit Depth (CMYK would be good but not required). This is already in GIMP, but not supported by everything. After GEGL is fully integrated this will be great.
Layer Masks and other nondestructive editing. Massively speeds up productivity and prototyping.
I'd really like an improvement to layers in GIMP. Right now you can't select multiple layers and move them up and down. Layer groups makes it tolerable, but it's still slower than photoshop.
On top of that the UI of GIMP is big and ugly. The menus are in akward places with way too much white space and padding taking up a good chunk of the screen. Some of this can be fixed with icon packs, customized panels, and a new theme, but it's always going to be a little oversized and awkward. Photoshop's menu organization is just about perfect; might as well copy it.