I've been working a lot with #glsl for a while now, writing complex code. Overall, I feel like the single improvement that would make the most difference for the language would be support for pass-by-reference instead of copy in/copy out syntax, as suggested here https://github.com/KhronosGroup/GLSL/issues/84
This would make it possible to do a lot of things that are either not possible or ridiculously inefficient to compile, including object oriented programming.
