I've been using GIMP for the last eight years now, and I still run up against:
* Bad text handling. Can't kern individual letters, just coarsely set tracking for the entire block; this is particularly bad given Linux's decidedly average font rendering.
* Fuzzy Selection needs a lot of manual cleanup.
* CMYK. I have to jump over to Adobe tools on the Macbook whenever I want to work with things destined for print.
I'm not a professional graphics/design guy; I'm a mostly-backend web developer who dabbles in graphics and print design on occasion. I want to love GIMP, I really do. :(
(I'm running GIMP 2.8 on Ubuntu 12.10; I've been a Linux user since Red Hat 7.2, so it's not a Noobuntu issue.)