I was in film school with John Knoll, who was really into special effects even then. At the time, there was no CGI, so many of PhotoShop's metaphors originated with the optical and photographic techniques used to perform, say, blue-screen traveling mattes. I remember seeing "this cool software my brother and I are working on" on a Mac II at ILM, doing a feathered, alpha-blended photo composite (giving a third eye to a photo of a baby) in the late 80s. At the time, the software was used to do background blending and stitching for CGI sequences in "The Abyss".
I have written on professional computer graphics software in the last few years, use the GIMP regularly in my applications-development work, and think PhotoShop still kicks it all 'round the block for fit and finish as well as raw capability. I agree with the earlier comments on Clayton Christensen, but don't think a product that tries to duplicate the market leader is the important one. I'd say perhaps that Picasa would be a better example there, quietly chomping away at the low end of PhotoShop's features for crop, scale, rotate, color-correct etc.