I do think artwork is the biggest issue to FLOSS games, but mainly because it's spread around everywhere, not that it doesn't exist.
There are loads of artists willing to create for free but there is not central repository for this artwork. We need a Launchpad or Github for textures, models, sounds, and maps.
Imagine a website where you can upload your artwork in standard open or common formats (so, for example, COLLADA, PNG/TGA, OGG) you could mark it with a license and you, or someone else, could create branches of that artwork, commit changes, view differences, tag with keywords - all the stuff we have with source code. That would be awesome.