wow, while this certainly sounds like a great idea, from an artists perspective I find it massively daunting.  I make textures and models, sometimes I turn them into tilesets, but if I had to learn a whole new metadata language to do so, I'd be much less likely to make any at all.  If this gets implemented, it better be easy for artists to understand and implemet, the idea of tagging in tiled seems good, but it still seems like a waste of my time, what would you prefer me to spend my tile on, applying massive metadata tags to existing art, or creating new art?   One sounds  fun, the other, not so much.

  It seems to me, as it so often does when working with FOSS games, that there needs to be a whole bunch of volunteers, who don't have artistic or programming skills but still want to help make games, that can be given these inbetweeny tasks. I really think a central job board type site, that lets programmers, artists and volunteers get together would be a major boon to the FOSS community.