GIMP's UI is alright for me, what I need are more features that are sorely lacking:
1) ability to work with images having an arbitrary number of spectral bands
2) floating-point pixel values
3) complex-valued pixels and their natural operations
4) images of arbitrary size, without need to open the whole image at once (which may not actually fit in memory)
5) save the processing graph and apply it to other images with a command line tool