Reading between the lines, my guess is that people are not chiming in because they think that their tiles may not conform to your individual vision and you will reject them, and that's not an environment they want to work in. Your role is the same as everyone else's... you can make suggestions, be they for things to draw, new tiles to include, or revisions to make, but you no more than anyone else have the right to decide what will stay, what will go, and what you will edit without warning.

ETA: Tell you what. What you're doing in making revisions, figuring out what goes together, matching styles, etc., is an important part of tileset creation, even community tileset creation, but it shouldn't be the first step. We don't yet know what the predominant visual style or instantiation of the theme will be, but I think the best way to find out is to let people contribute as they will for a while, and then start making decisions and more substantial edits a little later down the line when we find that we've got four sets of tiles in one direction and only one in another. Right now, though, most of us are still pretty close to concept art.