The API is great! Although having named arguments instead of "paramInt1", "paramInt2", etc. is counter productive... It would be better to have them named so we actually know what they are...
Aside from that, the World Edit script makes it possible to flatten a piece of land, etc., and put grass (well, make the area "green") where there is dirt, however when a player makes a nice slope (in dirt...) with the rake, it is not possible to make that slope "green" (in grass). Would it be possible to have a paint method? Instead of filling the area with data, just change the current blocks data to the specified value?
Something like :
world:paintTerrainInArea(c[1],c[2],c[3],c[4],c[5],c[6],c[7],c[8],c[9],c[10],c[11],c[12], blockID);