Let me see, here's the list of stuf gimp is still missing:
Native CMYK work and output - crucial for print
ability to add spot channels (again, for print - used for special dyes and lamination)
native 16 bit per channel support - crucial fro digital imaging and some esoteric tasks
Clipping layers - use one layer to mask another
vector masks on raster layers
Layer groups - introduced in latest beta
Layer effects - shadow, glow, storke - that change in real time according to layer data
vector layers (not paths - layers with vector data filled with color/pattern/gradient)
adjustment layers - layers that do non-destructive editing
smart objects - add external files as layers, scale them while keeping the original data
single, do-it-all transform tool
strong text tools (being addressed currently in gimp)
automatic layer boundary management
that's just off the top of my head, and I'm not even talking about interface, just functionality! I know both gimp and photoshop inside and out, know where every command is on both - Photoshop is so much ahead of Gimp it's absurd to even discuss both in the same article. Comparing Gimp and PS on simple tasks such as overlaying a couple of images is like taking a Lamborghini Gallardo out to the local corner store and back at 25mph and then claiming it performs just as well as your used Toyota Yaris.