The problem with Open Content is that it is concerned with end products, not source material, and is aimed at consumers, not producers. This reduces its value to both.
Free Software (Open Source), the inspiration for Open Content, ensures the free availability and free circulation of source code and supporting files. These are the materials that are used to make the program. If you want to change, extend, build on or borrow part of the source code existing program you can because it is the responsibility of everyone who works with the source code to make it publicly available.
Open Content only requires that the end product be made available. You can sample Open Content, remix it or compile it, but you don’t have access to the source materials used to make it as a guarantee of the license. If Open Content were software, the binaries (the runnable application) would be free, but the source code wouldn’t be released.
Open Content licenses must contain a guarantee to make source material available. The Midi files, the samples, the LaTeX or Docbook sources, the 3D models, the textures, the shaders, the patches, the PhotoShop or Gimp layered files, the graphical elements and scans, even the preparatory sketches, the written score, the script, the character and plot descriptions, unfinished versions of the work. All this will allow other people to build on Open Content rather than just to recycle it. Physical media are no limit. Free Software began when programs had to be distributed on bulky magnetic tapes. Open Content doesn’t necessarily need the Internet; photocopies of sketches and scores are much better than no source material at all.
Moving from Open Content to Open Source Material will enable a creative explosion in culture like the one that Free Software has enabled for programming. And for the same reason: shared source material improved and added to by a creative community that creators can build on.