Minecraft wiki entry for Pretty_Scary_Update, associated text: Lighting bugs present in 1.3.2 were only partially fixed. Dinnerbone first introduced changes in snapshots 12w39a/b. Dinnerbone tweeted to revert and push this to 1.5.[8][9]
Other changes postponed to 1.5 include redstone changes[10] and a rendering overhaul.[11][12]