I've had this discussion on slashdot before, mostly the one thing that keeps me from GIMP as a semi-pro photographer is the UI. I've used PhotoShop since the 7.0 debut up to CS3 and a brief poke at PS-CC, so I know the UI and where I expect to find stuff for my workflow... hell I could do the tasks I need to while asleep by now. GIMP on the other hand - and if you will excuse the graphic ( pun fully intended ) description - looks like the UI designers chowed down on all the UI elements and threw up on the screen. I've also used Linux, and GIMP, for many a year now and remember how much the GIMP devs hate listening to the users... we asked for how many years before they finally implemented single window mode?
SO the main problem isn't that GIMP is lacking features, it's that the UI is horrid to long time users of PS, leading to the "experts" not using it and thus leading to no one teaching the newbies to learn the program / UI. That's the main reason PS is taught in schools, it's what is known and used by the "experts".
For the inevitable slashdot car analogy: you can have two cars that can get from point A to point B along the same route in relatively the same timeframe, one that costs a fair bit to use but is an industry standard car like all others on the road ( PS ), or a free one that does nearly the same as the paid car but has bicycle handle bars for a steering wheel, a manual transmission with a reversed standard H shift pattern, the fuel pedal is on the handle bars and the brake is where the gas pedal usually is on normal cars, and you have to enter and exit through the trunk.... You can guess what vehicle 99% of people who have ever driven a normal vehicle will take.
Other than that, there is the non destructive editing and smart objects mentioned. I personally think the workflow for working with layers, clipping masks, and paths in PS is easier and more streamlined as well, but that is a personal opinion due to my workflow.