I think the barriers are quite often that collaboration around contributions happens in developer centric platforms. Am not sure what it is for GIMP, but often it's Github or IRC or some message board, which works really well for developer contributions, but then for things like UI design, marketing proposals, social media sign off etc... it's less than ideal.

I'd like to help contribute to some projects where I can lend a (non-development) hand, but being code-based projects, they don't feel set up to accept it.