- BSD-style licenses (of which I regard Apache v2 as the state-of-the-art) place no restriction on whether derived creations are returned to the commons. Thus the creative works of the community surrounding a commons created by a BSD-style license may be returned to that commons, may be applied to a different source-commons1 or may be incorporated into a closed-source work.
- GPL-style licenses require that derived creations, both resulting from the original commons and created newly around the commons, be licensed under the same license. The commons is thus enriched as it is used, but innovations created outside the commons can very easily be found to be licenseable only under the GPL and thus need to be compulsorily added to the commons - the artisan will often find that there is no freedom of choice in this regard.
- MPL-style licenses (of which I regard CDDL to be the state-of-the-art at present) require that creations derived from files in the commons be licensed under the same license as the original file, but allow newly-created files to be licensed under whatever license the creator of the file chooses. This is a win-win; the commons is continually enriched, and the artisan retains the freedom to license innovations in any way that's appropriate.
I think that view's still valid, but a view that explicitly recognizes that the MPL and CDDL do require giving back to the commons at some level is more complete in important ways. Thus, I like this three bucket taxomy.