Excellent and comprehensive Article!
Used photoshop and gimp for a long time but gradually transitioned to gimp as it evolved. One of the main reasons why I eventually moved to gimp is I work on both Linux and Windows so with Gimp, I don't have to switch between two different interfaces.
Honestly, there isn't much that I feel is lacking in gimp: It has the usual things you'd need for photo editing; layers, airbrush, clone/heal, curves, masks, fuzzy and foreground selection, and filters like Gaussian Blur, Unsharp Mask.
It also has some very good photo-related plugins like Wavelet Decompose for skin retouching, and Anisotropic+Patch-based smoothing for denoising (part of the GMIC plugin).
On the minus side, GIMP does not yet have adjustment layers, so you'll have to make do with things like extra layer copies and masks. It also lacks basic workflow features like non destructive editing.
Also look through this article- www.xp-pen.com/forum-1790.html , there are some great image editors, too.