For my image processing workflow there are two major features missing in current versions of GIMP: support for images with more than 8 bits per channel and adjustment layers. Today I decided to check out GIMP's non-stable, development version 2.9.1 (commit a545a4b) and I was positively surprised. Not only does it support 16-bpp (and more) but it is also multi-threaded. While still slower compared to the stable version running 8-bpp single-threaded, it is quite usable running on E6550@2.33GHz. Also, it finally has a real "overlay" layer mode, as opposed to "soft light pretending to be overlay" in the stable version. Here's an image I took in Shimbashi, Tokyo on my last year's trip to Japan, and processed today with GIMP. 16-bpp end to end.
Still, no adjustment layers... (2014-11-21 Simbashi, Tokyo, Japan)