While the OER community owes some of its genesis to the open source and free software movements, there are some aspects of how and why these movements work that I think are missing or need greater emphasis.
1. Its not what you share, its how you create it
One of the distinctive elements of the open source software movement are open development projects. These are the projects where software is developed cooperatively (not collaboratively, necessarily) in public, often by people contributing from multiple organisations. All the processes that lead to the creation and release of software – design, development, testing, planning – happen using publicly visible tools. Projects also actively try to grow their contributor base.
When a project has open and transparent governance, its much easier to encourage people to voluntarily provide effort free of charge that far exceeds what you could afford to pay for within a closed in-house project. (Of course, you have to give up a lot of control, but really, what was that worth?)