Disclaimer: most of my contributions for OSS have been on a paid basis. I choose to contribute to open source because the projects may have people that think similarly in terms of approaching problems. It's also a good way to learn and apply knowledge that impacts other peoples' work instead of your own.

My contributions are responses to issues that are clearly laid out and have a clear target spec. Vague issues that have a large number of unknowns are ones I would be more reluctant in approaching.