My biggest issue a lot of times with "photoshop killers", is that they rarely tackle non-destructive editing (I believe gimp is included on this, but I haven't touched it for about a year now). Many have masking capabilities, but any lighting adjustments to an image or corrections in general are destructive edits (meaning they alter the underlying pixels of the source image). Non destructive edits are extremely important, especially if a client asks for changes later. It's much easier to change a slider on an adjustment layer than to undo a ton of your work because you can't return the original detail to a picture. When I'm doing a photomanipulation, I'll usually have a minimum of three adjustment layers on a source image, allowing me to fine tune the look I need. In alternatives, I can't do that. Once I apply a curve change, I cant tell Krita, "hey remember that curve adjustment I did 14 steps ago? Instead of that one, can you do a similar adjustment except with less highs in the red channel?" In photoshop I can. That's always been the major difference for me.