Minecraft wiki entry for Development_versions, associated text: Added support for directional opacity of blocks
Used by carpet, enchanting table, End portal frame, farmland, grass paths, extended piston base blocks, slabs, snow layers, and stairs: their full-block faces block light, and their non-full faces let light through the gaps.
Not used by cauldrons, comparators, daylight detectors, hoppers, redstone repeaters, or stonecutters: these are fully transparent, and let light through all their faces, even their full-block faces.
Added loot table for illusioners.