The biggest revelation for me when I switched to Photoshop for work about 4 years ago is that non-destructive editing is sooooo much nicer.
I always had dozens of "backup" layers in my years with Gimp just in case I messed something up. I was always cautious about the order in which you had to do things. I was amazed with photoshop at the fact that you could edit text after warping, gradient coloring and outlining it. Saved so much hassle.
I read non-destructive is in the pipelines for Gimp, and that would finally make it start become a viable alternative again.