Thank you @David_Tschumperle, @paperdigits, @Phil_Howcroft, @Raja for all the feedback!
While I can’t anticipate all of the types of questions that may be asked, I think we can cover the bigger points well.
- Freedom - an obvious first point, but one that may be unfamiliar to proprietary software users/beginners that may not know any better.
- Control - in tandem with freedom is the control one has over their entire image processing stack if they want.
- Functionality - Some free software does things that has no proprietary version.
- Cost - Not always the biggest motivator for long-time users but certainly a lower barrier to entry for new users when price is not an issue for obtaining quality software.
- Community/Involvement - Free software communities are usually quite active places where users can interact with other user as well as the folks doing actual development of the tools.
As @Raja noted about only hearing about GIMP in terms of a comparison to Ps, I’m thinking perhaps of making a stronger point for learning/thinking about processing in terms of results and processes as opposed to particular software steps. That is, knowing how an Unsharp Mask works and where it might be appropriate is better than simply being told “Apply an Unsharp Mask with these settings”.